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★ 9/10

A Weighted Product Bound

Positive reals a1,…,a100a_1, \dots, a_{100} satisfy ai≥a101−ia_i \ge a_{101-i} for i=1,…,50i = 1, \dots, 50. Define

xk=k ak+1a1+a2+⋯+ak(k=1,…,99).x_k = \frac{k\,a_{k+1}}{a_1 + a_2 + \cdots + a_k} \qquad (k = 1, \dots, 99).

P…

sequencesAM-GMinequality
★ 9/10

Two Zeros and a Power Bound

Let aa be a constant and

f(x)=x(eax+a)+ln⁡x−1.f(x) = x\left(e^{ax} + a\right) + \ln x - 1.

1. Discuss the monotonicity of f(x)f(x). 2. Suppose f(x)f(x) has two zeros x1<x2x_1 < x_2.

  1. Find the ran…
functionscalculusSubstitutionMonotonicity+1
★ 9/10

Conservative Sequences

A finite sequence {an}\{a_n\} with k⩾2k \geqslant 2 terms is called conservative if

∣ai−ai−1∣⩽2for all 2⩽i⩽k.|a_i - a_{i-1}| \leqslant 2 \qquad \text{for all } 2 \leqslant i \leqslant k.

Let TmT_m be …

combinatoricsCountingRecursionsequences+1
★ 9/10

Chebyshev Coefficient Sum

Let f(x)=ax3+bx2+cx+df(x) = ax^3 + bx^2 + cx + d with a≠0a \neq 0, and suppose ∣f(x)∣⩽1|f(x)| \leqslant 1 for all x∈[−1,1]x \in [-1, 1]. Find the maximum possible value of

∣a∣+∣b∣+∣c∣+∣d∣.|a| + |b| + |c| + |d|.
Absolute ValuealgebraPolynomialsEstimation+1
★ 9/10

A Piecewise Polynomial Ladder

A function f(x)f(x) on (0,+∞)(0, +\infty) is defined piecewise: for n−1<x⩽nn - 1 < x \leqslant n (with n∈N∗n \in \mathbb{N}^*),

f(x)=(x−n+1)(x−n)n.f(x) = (x - n + 1)(x - n)^n.

Determine, with proof, which o…

functionsExtremacalculusEstimation+1
★ 9/10

Reordering a Geometric Sequence

A finite geometric sequence {an}\{a_n\} has NN terms (N⩾3N \geqslant 3), first term a1>0a_1 > 0, and common ratio q∈(0,1)q \in (0, 1). Its terms are rearranged into a sequence {bn}\{b_n\} suc…

combinatoricsGeometric ProgressionCountingRecursion+1
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