Apply the course version boundaries in notes.md; HSK tasks are transferable language practice, not a claimed HSK 3.0 mock.
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gre_mathematics:mixed-revision:1
A linear map on R³ has rank 2. Find its nullity and explain what this means.
Jawaban dan penjelasan
Nullity 1 by rank-nullity. Its kernel is a one-dimensional subspace.
gre_mathematics:mixed-revision:2
Evaluate the counterclockwise integral of 4/(z+1) on |z+1|=2.
Jawaban dan penjelasan
8πi: one enclosed simple pole at −1 has residue 4.
gre_mathematics:mixed-revision:3
Solve y′=−2y with y(0)=5 and verify the result by differentiation.
Jawaban dan penjelasan
y=5e^(−2t); derivative is −10e^(−2t)=−2y and initial value is 5.
gre_mathematics:mixed-revision:4
In Z/24Z under addition, find the order of 9 and list the subgroup it generates. How many cosets does it have?
Jawaban dan penjelasan
The order is 24/gcd(9,24)=8. The subgroup is {0,3,6,9,12,15,18,21}; it has index 24/8=3.
gre_mathematics:mixed-revision:5
The additive map Z/15Z to Z/15Z sends x to 5x. Find its kernel and image and explain why the quotient by the kernel is not the entire codomain.
Jawaban dan penjelasan
The kernel is {0,3,6,9,12}, the image is {0,5,10}, and the quotient has three elements. The codomain has fifteen elements; the map is not onto.
gre_mathematics:mixed-revision:6
In Z/9Z, list the units and nonzero zero divisors. Explain why the ring is not a field despite having an identity.
Jawaban dan penjelasan
Units are 1,2,4,5,7,8. Nonzero zero divisors are 3 and 6. Since 3·3=0 modulo 9 and 3 has no inverse, not every nonzero element is a unit.
gre_mathematics:mixed-revision:7
Compare 2Z as a submodule of the Z-module Z with 2Z as a proposed subspace of Q. Give a scalar action that distinguishes them.
Jawaban dan penjelasan
2Z is closed under integer scaling and addition, so it is a Z-submodule. It is not a Q-vector subspace because rational scalar 1/2 sends 2 to 1, which is not in 2Z.
gre_mathematics:mixed-revision:8
Decide whether x²+1 is irreducible over Q, R and F2, explaining each change of coefficient field.
Jawaban dan penjelasan
Over Q and R it has no root and is an irreducible quadratic. Over F2 it is (x+1)² and is reducible. The real field differs from the complex field, over which it splits.
gre_mathematics:mixed-revision:9
A finite field extension L/K has degree 10. Can an element of L have minimal polynomial of degree 3 over K?
Jawaban dan penjelasan
No. The intermediate field K(alpha) would have degree 3, and the tower law would require 3 to divide 10.
gre_mathematics:mixed-revision:10
Solve 8x≡4 modulo 12, listing every residue solution in the range 0 through 11.
Jawaban dan penjelasan
gcd(8,12)=4, so reduction gives 2x≡1 modulo 3. Thus x≡2 modulo 3 and the four original residues are 2,5,8,11.
gre_mathematics:mixed-revision:11
Find the least nonnegative solution to x≡3 modulo 5 and x≡4 modulo 7. State the modulus of uniqueness.
Jawaban dan penjelasan
Write x=3+5k; 5k≡1 modulo 7 gives k≡3. Therefore x≡18 modulo 35; 18 is the least nonnegative solution.
gre_mathematics:mixed-revision:12
Determine whether f_n(x)=x/n converges uniformly to zero on [0,2] and on R.
Jawaban dan penjelasan
On [0,2], the supremum error is 2/n, so convergence is uniform. At each fixed real x the limit is still zero, but on R the error is unbounded for every n, so convergence is not uniform there.
gre_mathematics:mixed-revision:13
Use the Weierstrass M-test to decide uniform convergence of sum sin(nx)/n² on R. Explain why replacing n² by n defeats this argument.
Jawaban dan penjelasan
Each absolute term is at most 1/n² and that numerical series converges, so the function series converges uniformly and absolutely. The bound 1/n has a divergent sum, making this M-test inconclusive; that failure alone is not a proof of divergence.
gre_mathematics:mixed-revision:14
Find the interior, closure and boundary of the rationals Q as a subset of R.
Jawaban dan penjelasan
The interior is empty because every interval contains irrational points; the closure is R because rationals are dense. Thus the boundary is all of R.
gre_mathematics:mixed-revision:15
In the space X=(0,1), consider the whole set X. Is it relatively closed and relatively open? Is it compact in its usual metric?
Jawaban dan penjelasan
The whole space is both open and closed relative to itself. It is not compact; the sequence 1/n for n≥2 has no subsequence converging to a point in X. Relative closedness alone does not provide Euclidean closedness in R.
gre_mathematics:mixed-revision:16
Find the radius and complete real interval of convergence of sum x^n/(n·2^n), n≥1.
Jawaban dan penjelasan
Radius 2. At −2 the alternating harmonic series converges, while at 2 the harmonic series diverges; the interval is [−2,2).
gre_mathematics:mixed-revision:17
Give the degree-three Maclaurin polynomial for sin x and an absolute error bound for |x|≤0.2 using a degree-three Taylor remainder.
Jawaban dan penjelasan
The polynomial is x−x³/6. The fourth derivative is sin x and has absolute value at most 1, so the Lagrange bound is |x|⁴/24≤0.000066667. A sharper bound is possible, but is not needed to justify this one.
gre_mathematics:mixed-revision:18
Determine whether the integral of 1/x² from −1 to 1 exists as an ordinary improper integral or as a finite symmetric principal value.
Jawaban dan penjelasan
Both fail. The integral diverges positively on each side of zero; symmetric cutoffs give 2/epsilon−2, which tends to infinity rather than cancelling.
gre_mathematics:mixed-revision:19
Rotate the region between y=2 and y=x on 0≤x≤2 around the x-axis. Find its volume using washers.
Jawaban dan penjelasan
Outer radius 2 and inner radius x give V=π integral 0..2 (4−x²) dx=π(8−8/3)=16π/3.
gre_mathematics:mixed-revision:20
Classify the origin for f=x²+4xy+y² and g=x⁴+y⁴, explaining which Hessian test is decisive.
Jawaban dan penjelasan
For f, f_xx=f_yy=2 and f_xy=4, so D=4−16=−12; it is a saddle. For g the Hessian test is inconclusive, but g≥0 with equality only at the origin proves a strict global minimum.
gre_mathematics:mixed-revision:21
Find the maximum and minimum of x+y on x²+y²=8, stating the points where they occur.
Jawaban dan penjelasan
(x+y)²≤2(x²+y²)=16, so the range is [−4,4]. Maximum 4 occurs at (2,2), minimum −4 at (−2,−2). The compact circle ensures extrema are attained.
gre_mathematics:mixed-revision:22
Use the divergence theorem to compute the outward flux of F=(x,2y,3z) through the unit cube 0≤x,y,z≤1.
Jawaban dan penjelasan
div F=1+2+3=6. The closed cube has volume 1, so total outward flux is 6. Opposite face orientations must be respected in a direct calculation.
gre_mathematics:mixed-revision:23
Evaluate the clockwise unit-circle circulation of F=(−y,x). Explain why Green's theorem cannot be applied over the full disk to F=(−y/(x²+y²),x/(x²+y²)).
Jawaban dan penjelasan
For the first field, counterclockwise circulation is 2π; clockwise is −2π. The second field is undefined at the origin and lacks the smoothness hypothesis on the full disk, even though its curl vanishes away from zero.
gre_mathematics:mixed-revision:24
For f(x)=3x−4 on R, find its inverse and decide whether f is an involution.
Jawaban dan penjelasan
The inverse is (y+4)/3. It is not an involution: f(f(x))=9x−16 is not identically x.
gre_mathematics:mixed-revision:25
Let g(x)=2/x on R excluding zero. Verify the inverse composition and evaluate g⁹(4).
Jawaban dan penjelasan
g(g(x))=2/(2/x)=x with every input nonzero. Odd iterates equal g, so g⁹(4)=1/2.
gre_mathematics:mixed-revision:26
Give a function and disjoint nonempty input sets whose images intersect. State a condition on the function that rules this out.
Jawaban dan penjelasan
Use f(x)=x², A={−3}, B={3}; both images are {9}. Injectivity ensures equal outputs require equal inputs, so disjoint input sets then have disjoint images.
gre_mathematics:mixed-revision:27
Negate the claim that every integer has an integer strictly larger than it. Distinguish the negation from a counterexample to the original claim.
Jawaban dan penjelasan
The negation is: there exists an integer x such that every integer y satisfies y≤x. The original is true because y=x+1 is always an integer; its negation is false, and no counterexample to the original exists.
gre_mathematics:mixed-revision:28
P(A)=0.6, P(B)=0.4 and P(A∩B)=0.24. Find both conditional probabilities and test independence.
Jawaban dan penjelasan
P(A|B)=0.6; P(B|A)=0.4. The joint probability is the product 0.6·0.4, so the events are independent.
gre_mathematics:mixed-revision:29
A sample mean has standard error 4 at n=9 for iid observations. How large must n be to reduce standard error to 1?
Jawaban dan penjelasan
Population standard deviation is 4√9=12. Solve 12/√n=1 to obtain n=144; a fourfold precision gain needs sixteen times as many observations.
gre_mathematics:mixed-revision:30
A similar model uses 80% of every original length. Find the area and volume decreases as percentages.
Jawaban dan penjelasan
Length factor 0.8 gives area factor 0.64 and volume factor 0.512. The decreases are 36% and 48.8%, respectively.
gre_mathematics:mixed-revision:31
Foci are (−5,0),(5,0). Classify the locus r_left+r_right=14 and the locus r_left−r_right=−6; specify any branch.
Jawaban dan penjelasan
The first is an ellipse with a=7,c=5,b²=49−25=24: x²/49+y²/24=1. The second is a hyperbola x²/9−y²/16=1 restricted to x≤−3, because the signed difference is negative.
gre_mathematics:mixed-revision:32
For y=3 cos(4x−π), give the amplitude, period and horizontal shift. Find one maximum location.
Jawaban dan penjelasan
Amplitude 3; period π/2; right shift π/4. One maximum is x=π/4, where the argument is zero.
gre_mathematics:mixed-revision:33
For x=t,y=t² with t in [−1,1], give the traced set, direction and slope at t=−1/2.
Jawaban dan penjelasan
Only the parabola segment y=x², −1≤x≤1, is traced. It moves from (−1,1) through (0,0) to (1,1). The slope is 2t, hence −1 at t=−1/2.
gre_mathematics:mixed-revision:34
Find the projection of (2,1) onto the line spanned by (1,1), and verify its remainder is orthogonal.
Jawaban dan penjelasan
Coefficient is (2+1)/(1+1)=3/2, so projection is (3/2,3/2). Remainder (1/2,−1/2) has dot product zero with (1,1).
gre_mathematics:mixed-revision:35
Compute signed and unsigned volumes for edges (1,0,0),(0,2,0),(0,0,3). What changes if the last two edges are exchanged?
Jawaban dan penjelasan
The scalar triple product is 6 and volume is 6. Exchanging the last two edges changes the signed result to −6, while geometric volume remains 6.
gre_mathematics:mixed-revision:36
Solve y′+2y=6 with y(0)=1 and check the equation.
Jawaban dan penjelasan
Integrating factor e^(2x) gives y=3+C e^(−2x); C=−2, so y=3−2e^(−2x). Its derivative is 4e^(−2x), and y′+2y=6.
gre_mathematics:mixed-revision:37
Give the general solution of y″−2y′+y=e^x, including the resonant particular term.
Jawaban dan penjelasan
The repeated characteristic root is 1, giving (C₁+C₂x)e^x. A particular solution is x²e^x/2, because (D−1)²(e^x z)=e^x z″ and z″=1. Add both contributions.
gre_mathematics:mixed-revision:38
Count allocations of 10 identical tokens to 3 named boxes when the first gets at least 2 and the others may be empty.
Jawaban dan penjelasan
Subtract the first-box lower bound 2. There are 8 remaining stars and 2 bars, so C(10,2)=45 allocations.
gre_mathematics:mixed-revision:39
Explain why S=m² is preserved when S increases by B and then B increases by 2, starting S=0,B=1. Give the state after four iterations.
Jawaban dan penjelasan
Before an iteration B=2m+1. Updating S gives m²+2m+1=(m+1)², then B becomes 2(m+1)+1. After four iterations S=16,B=9; B is the next addend, not the last one.
gre_mathematics:mixed-revision:40
For x=u+2v,y=3u+v, compute v_x and u_y.
Jawaban dan penjelasan
J=[[1,2],[3,1]] has determinant −5; inverse is [[−1/5,2/5],[3/5,−1/5]]. Therefore v_x=3/5 and u_y=2/5.
gre_mathematics:mixed-revision:41
Why does the map (u,v)↦(u³,v) contradict the claim that zero Jacobian determinant means no inverse exists?
Jawaban dan penjelasan
It is bijective on R² with inverse (x,y)↦(real cube root of x,y). Its determinant 3u² vanishes at u=0, where the inverse fails to be differentiable. Nonzero determinant is a sufficient differentiable-local-inverse condition, not a necessary condition for mere invertibility.
gre_mathematics:mixed-revision:42
For L=[[1,0],[2,1]], U=[[3,1],[0,2]], b=(7,20), solve LUx=b.
Jawaban dan penjelasan
Forward y₁=7,y₂=20−14=6. Backward x₂=3,x₁=(7−3)/3=4/3. Verify Ux=(7,6) and Ly=(7,20).
gre_mathematics:mixed-revision:43
Maps A,B:R⁵→R⁵ have nullities 3 and 1, respectively. Find bounds on nullity AB and explain why it cannot be 1.
Jawaban dan penjelasan
Rank B=4. The intersection of im B (dimension 4) with ker A (dimension 3) has dimension at least 4+3−5=2 and at most 3. Therefore nullity AB ranges from 1+2=3 to 1+3=4; the ambient dimension forces overlap.
gre_mathematics:mixed-revision:44
Reverse and evaluate ∫₀¹∫ₓ¹ e^(2y)dy dx.
Jawaban dan penjelasan
Region 0≤x≤y≤1 gives ∫₀¹y e^(2y)dy. Integration by parts gives [y e^(2y)/2−e^(2y)/4]₀¹=(e²+1)/4. The factor y comes from the inner x-interval length.
gre_mathematics:mixed-revision:45
Evaluate ∫₀^π sin(8x)/sin x dx and justify treatment of both endpoints.
Jawaban dan penjelasan
Reflection x↦π−x reverses the integrand sign. The endpoint limits are 8 and −8, so singularities are removable and the integral is 0.
gre_mathematics:mixed-revision:46
A sphere has R=2, depth h=1 increasing at 1/2 per time unit. Find the signed volume rate.
Jawaban dan penjelasan
Current section area is π(4−1)=3π, so signed dV/dt=3π/2, positive for filling.
gre_mathematics:mixed-revision:47
For f(x)=sin(4x),g(x)=5x, give limits of f/g and (f²−f)/(2g−g³) as x→0.
Jawaban dan penjelasan
The first is f′(0)/g′(0)=4/5. The second is −f′(0)/(2g′(0))=−2/5 after factoring, with all nearby denominators checked.
gre_mathematics:mixed-revision:48
In the divisor basis on integers at least 2, compute closure of {12} and test membership of 6 and 36.
Jawaban dan penjelasan
Closure is {12,24,36,…}. The point 36 belongs; 6 does not, because U_6 excludes 12.
gre_mathematics:mixed-revision:49
For d(x,y)=|x³−y³| on R, is x_n equal to the cube root of n a Cauchy sequence? Explain using the image.
Jawaban dan penjelasan
No. Its image is n, not a Cauchy sequence in ordinary R. The space is complete, but completeness does not make every sequence Cauchy.
gre_mathematics:mixed-revision:50
Find the residue of e^(2z)/(z−1)³ at z=1 and its positively oriented integral around a circle centred at 1 containing no other singularity.
Jawaban dan penjelasan
For a triple pole, residue is g″(1)/2!=4e²/2=2e². The contour integral is 2πi times the residue, or 4πi e².
gre_mathematics:mixed-revision:51
Count conjugacy classes in S5 by listing the partitions of 5.
Jawaban dan penjelasan
The partitions are 5, 4+1, 3+2, 3+1+1, 2+2+1, 2+1+1+1 and 1+1+1+1+1. There are seven classes. Orders alone do not distinguish every type.
gre_mathematics:mixed-revision:52
In a general ring where every element is idempotent, prove 2a=0 without assuming that distinct elements commute.
Jawaban dan penjelasan
Apply idempotence to a+a: (a+a)²=a+a. Expanding gives 4a²=2a; since a²=a, subtract 2a to obtain 2a=0. Only distributivity and additive-ring operations were used.
gre_mathematics:mixed-revision:53
Primitive eighth roots are the roots of x⁴+1. Find their sum and product, and justify why these roots have exact order eight.
Jawaban dan penjelasan
Their fourth power is −1, so their eighth power is 1 and their order cannot divide 4. The divisors of 8 are 1,2,4,8, so their order is 8. Vieta gives sum 0 and product 1.