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6/10

Range of a Coefficient Sum on a Circular Arc

figure

In sector AOBAOB the central angle is 120120^\circ. PP is a point on arc ABAB, and $\overrightarrow{OP} = x,\overrightarr…

plane geometryExtremaAM-GMvectors
6/10

Path of a Midpoint Between Two Equilateral Triangles

figure

Given AB=10AB = 10, point MM is a moving point on segment ABAB. On the same side of ABAB, equilateral triangles…

plane geometryParametrizationanalytic geometryTriangle Geometry
8/10

Equal Triangle Areas on a Conic

In the coordinate plane xOyxOy, let BB be the reflection of A(1,1)A(-1,1) across the origin, and let PP be a moving point such that the product of the slopes of APAP and BPBP is $-\df…

conic sectionsCaseworkSymmetry
6/10

A Shiftable Even Function

Let ωR\omega \in \mathbb{R} and f(x)=(x6)2sin(ωx)f(x) = (x - 6)^2 \sin(\omega x). Suppose there exists a constant aRa \in \mathbb{R} such that f(x+a)f(x + a) is an even function. Determine which of t…

functionstrigonometryTrigonometric IdentitiesSymmetry
7/10

An e\mathrm{e} Bound, Refined

Let aRa \in \mathbb{R} and

f(x)=(1+ax)(1+1x)x,x>0.f(x) = \left(1 + \frac{a}{x}\right)\left(1 + \frac{1}{x}\right)^x, \qquad x > 0.

1. When a=1a = 1, prove that f(x)>ef(x) > \mathrm{e}. 2. If $f(x) > \m…

calculusMonotonicityEstimationLogarithms+1
9/10

An Injectivity Condition

Find all complex numbers α\alpha such that for any distinct z1,z2z_1, z_2 with z1,z2<1|z_1|, |z_2| < 1,

(z1+α)2+αz1(z2+α)\left(z_1 + \alpha\right)^2 + \alpha\overline{z_1} \ne \left(z_2 + \alpha\right)…
complex numbersAbsolute ValueEstimation
8/10

An Alternating Coefficient Recursion

Sequences {an}\{a_n\} and {bn}\{b_n\} satisfy

bnan+an+1+bn+1an+2=0,bn=3+(1)n2,b_na_n + a_{n+1} + b_{n+1}a_{n+2} = 0, \qquad b_n = \frac{3 + (-1)^n}{2},

with a1=2a_1 = 2 and a2=4a_2 = 4.

1. Find a3,a4,a5a_3, a_4, a_5. 2.…

Geometric ProgressionRecursionsequencesTelescoping+1
7/10

Squeezed by a Recurrence

A sequence {an}\{a_n\} satisfies a1=3a_1 = 3 and

an2(1+an+1)an+2=0(nN).a_n^2 - (1 + a_{n+1})\,a_n + 2 = 0 \qquad (n \in \mathbb{N}^*).

1. Prove that 2<an+1<an2 < a_{n+1} < a_n for all nn. 2. Let $S_n = a_1…

sequencesinequality
6/10

A Root-of-Unity Shift

A complex number zz satisfies z=1|z| = 1 and

zn=z+2.z^n = z + \sqrt{2}.

Find the least positive integer nn for which such a zz exists.

complex numbersModular ArithmeticCasework
7/10

An Isomorphic Inequality

If the inequality

mxemx2lnxmx\mathrm{e}^{mx^2} \geqslant \ln x

holds for all xx in the domain, find the range of possible values of the real number mm.

ExtremacalculusSubstitutionMonotonicity+1
7/10

A Partial Weighted Coefficient Sum

Let (1+x)50=a0+a1x+a2x2++a50x50(1+x)^{50}=a_0+a_1x+a_2x^2+\cdots+a_{50}x^{50}. Find the value of

a1+2a2+3a3++25a25.a_1+2a_2+3a_3+\cdots+25a_{25}.
combinatoricsalgebraBinomial TheoremSymmetry
7/10

A Tangent from a Focal Point

Let FF be the right focus of the ellipse x25+y2=1\dfrac{x^2}{5} + y^2 = 1, and let MM be the point of the ellipse in the first quadrant with MFMF perpendicular to the xx-axis. The lin…

Tangent CircleCirclesconic sectionsanalytic geometry

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