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8/10

A Linear System Limit

Sequences {xn},{yn}\{x_n\}, \{y_n\} satisfy x0=5x_0 = 5, y0=2y_0 = 2, and

xn+1=72xn+6yn,yn+1=3xn+5yn.x_{n+1} = -\frac{7}{2}x_n + 6y_n, \qquad y_{n+1} = -3x_n + 5y_n.

Find limnxn\lim_{n\to\infty} x_n and $\lim_{n\to\i…

Geometric ProgressionLimitsRecursionsequences
7/10

A Telescoping Tangent Sum

Given tan4x=33\tan 4x = \dfrac{\sqrt3}{3}, evaluate

sin4xcos8xcos4x+sin2xcos4xcos2x+sinxcos2xcosx+sinxcosx.\frac{\sin 4x}{\cos 8x\cos 4x} + \frac{\sin 2x}{\cos 4x\cos 2x} + \frac{\sin x}{\cos 2x\cos x} + \frac{\sin x}{\cos x}.
trigonometryTrigonometric IdentitiesTelescoping
8/10

An Area Range from a Parallel Chord

The ellipse C:x2a2+y2b2=1C:\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}=1 (a>b>0a>b>0) is given. A line ll passes through the origin, and a line parallel to ll meets the ellipse CC at two points AA and…

Affine Transformationconic sectionsMenelaus's Theorem
7/10

A Quadratic Form Window

Real numbers a,ba, b satisfy

a2ab2b2=1.a^2 - ab - 2b^2 = 1.

Find the range of possible values of a2+b2a^2 + b^2.

SubstitutionalgebraAM-GMinequality
6/10

A Diagonal Dot Identity

In quadrilateral ABCDABCD, the diagonals satisfy AC=aAC = a and BD=bBD = b. Find

\left(\overrightarrow{AB} + \overrightarrow{DC}\right)\cdot\left(\overrightarrow{BC} + \overrightarrow…
plane geometryPolarization Identityvectors
8/10

Powers Two Apart

Find the number of positive integer solutions (x,y,u,v)(x, y, u, v) of

2x3y2u5v=2.\left|2^x 3^y - 2^u 5^v\right| = 2.
Diophantine Equationsnumber theoryModular ArithmeticCasework
7/10

A Rectangle in an Ellipse?

Let A,B,CA, B, C be three points on the ellipse x2a2+y2b2=1\dfrac{x^2}{a^2} + \dfrac{y^2}{b^2} = 1 (a>b>0a > b > 0), and let OO be the origin. Can the quadrilateral OABCOABC be a rectangle? Justif…

Parametrizationconic sectionsanalytic geometryDiscriminant
8/10

Two Solutions of a Mixed Equation

The equation

ex+m(e2)lnxln2x(m+2)=0\mathrm{e}^{x+m} - (\mathrm{e} - 2)\ln x - \ln^2 x - (m + 2) = 0

has exactly two real solutions. Find the range of mm.

functionsExtremacalculusMonotonicity+1
7/10

Derivative Symmetries

Differentiable functions f(x),g(x)f(x), g(x) on R\mathbb{R} satisfy

f(x+3)=g(x)+4,f(x)+g(1+x)=0,f(x + 3) = g(-x) + 4, \qquad f'(x) + g'(1 + x) = 0,

and g(2x+1)g(2x + 1) is an even function. Determine, with proof, …

functionscalculusFunctional EquationsSymmetry
5/10

Where to Build the Dock

figure

As shown in the figure, an aquaculture center is located on an island AA. Two towns BB and CC lie on a straight coas…

Law of SinestrigonometryLaw of Cosines
8/10

Growth of a Recursion

Let a1=1a_1 = 1, a2=8a_2 = 8, and

an+1=an1+4nan(n=2,3,).a_{n+1} = a_{n-1} + \frac{4}{n}a_n \qquad (n = 2, 3, \ldots).

1. Prove that there exists c>0c > 0 with ancn2a_n \leqslant cn^2 for all nn. 2. Prov…

RecursionsequencesInductionTelescoping+1
6/10

A Quadratic-Form Maximum

Real numbers x,yx, y satisfy

4x2+y2+xy=1.4x^2 + y^2 + xy = 1.

Find the maximum value of 2x+y2x + y.

SubstitutionalgebrainequalityDiscriminant

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