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★ 7/10

A Min of Three Quantities

Let x,y,z>0x, y, z > 0. Find the maximum value of

min⁡{2x, 1y, y+1x}.\min\left\{2x,\ \frac{1}{y},\ y + \frac{1}{x}\right\}.
algebraAM-GMinequality
★ 7/10

Midpoints of Reflected Chords of a Parabola

figure

As shown in the figure, let C: y2=2xC:\ y^2 = 2x be a parabola, and let ll be a line through the point A(2,0)A(2,0) that int…

ExtremaParametrizationconic sectionsAM-GM+1
★ 5/10

Counting Disjoint Pairs

Let MM be a set with 66 elements, and define

T={(A,B)∣A,B⊆M, A≠∅, B≠∅, A∩B=∅}.T = \left\{(A, B) \mid A, B \subseteq M,\ A \neq \varnothing,\ B \neq \varnothing,\ A \cap B = \varnothing\right\}.

Find the nu…

combinatoricsCountingSet Theory
★ 7/10

A Median Difference Condition

In △ABC\triangle ABC, MM is the midpoint of BCBC, BM=2BM = 2, and AM=AB−ACAM = AB - AC. Find the maximum possible area of △ABC\triangle ABC.

plane geometryExtremaconic sectionsTriangle Geometry
★ 7/10

A Diameter Circle and a Fixed Length

An ellipse x2a2+y2b2=1\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}=1 (a>b>0a>b>0) passes through the point P(62,12)P\left(\dfrac{\sqrt 6}2,\dfrac 12\right) and has eccentricity 22\dfrac{\sqrt 2}2. Let M(2,t)M(2,t)…

Circlesconic sectionsanalytic geometry
★ 8/10

A Floor-Sqrt Recurrence Limit

A sequence {xn}\{x_n\} satisfies x1=1x_1=1 and xn+1=xn+⌊xn⌋x_{n+1}=x_n+\lfloor\sqrt{x_n}\rfloor. Find

lim⁡n→+∞xnn2.\lim_{n\to+\infty}\frac{x_n}{n^2}.
LimitscalculussequencesEstimation+1
★ 7/10

A Circumcenter Identity

Let OO be the circumcenter of △ABC\triangle ABC with A=π3A = \dfrac{\pi}{3}, and suppose

cos⁡Bsin⁡CAB→+cos⁡Csin⁡BAC→=2m\ov…\frac{\cos B}{\sin C}\overrightarrow{AB} + \frac{\cos C}{\sin B}\overrightarrow{AC} = 2m\ov…
Law of SinestrigonometryTrigonometric Identitiesvectors
★ 6/10

Trapped Between Two Constraints

Positive reals a,ba, b satisfy

2a+2b≤15and4a+3b≤2.2a + 2b \le 15 \qquad\text{and}\qquad \frac{4}{a} + \frac{3}{b} \le 2.

Find the range of 3a+4b3a + 4b.

Cauchy-Schwarzinequality
★ 8/10

A Tangent Intersection Locus

Let P(x0,y0)P(x_0, y_0) be a point inside the ellipse E ⁣:x2a2+y2b2=1E \colon \dfrac{x^2}{a^2} + \dfrac{y^2}{b^2} = 1 (with a>b>0a > b > 0). A line ll through PP meets the ellipse at AA and BB, and …

Pole and PolarExtremaconic sectionsanalytic geometry
★ 7/10

Acute Angles and a Parameter

Suppose there exist α,β\alpha, \beta with α\alpha, β\beta, and α+β\alpha + \beta all acute, satisfying

sin⁡α2=kcos⁡β.\sin\frac{\alpha}{2} = k\cos\beta.

Find the range of possible values of…

trigonometryExtremaMonotonicity
★ 5/10

An Exponential Minimum Bound

Let f(x)=ex−xf(x) = e^x - x.

1. Find the equation of the tangent line to y=f(x)y = f(x) at (0,f(0))(0, f(0)). 2. Find the extreme values of f(x)f(x). 3. Let g(x)=f(x)+x2+xg(x) = f(x) + x^2 + x. Prove that the m…

ExtremacalculusTangent Line Trick
★ 7/10

Tangents from the Center

Let P(a,b)P(a, b) be any point on the ellipse x23+y22=1\dfrac{x^2}{3} + \dfrac{y^2}{2} = 1. From the origin OO, two tangent lines to the circle

(x−a)2+(y−b)2=65…(x - a)^2 + (y - b)^2 = \frac{6}{5} \qquad …
Circlesconic sectionsanalytic geometryVieta's Formulas

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