A geometry problem by Aakhyat Singh

Geometry Level 3

{ csc A sin A = a 3 sec A cos A = b 3 \large \begin{cases} \csc A-\sin A=a^3 \\ \sec A - \cos A =b^3 \end{cases}

Given the above, which of options is correct?

a 2 b 2 = a 2 + b 2 a^2b^2=a^2+b^2 None of the others a 3 + b 3 = a b a^3+b^3=ab a 2 b 2 = 1 a 2 + b 2 a^2b^2=\frac 1{a^2+b^2}

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1 solution

Chew-Seong Cheong
Aug 17, 2017

{ csc A sin A = a 3 a 3 = 1 sin A sin A = 1 sin 2 A sin A = cos 2 A sin A sec A cos A = b 3 b 3 = 1 cos A cos A = 1 cos 2 A cos A = sin 2 A cos A \begin{cases} \csc A - \sin A = a^3 & \implies a^3 = \dfrac 1{\sin A} - \sin A = \dfrac {1-\sin^2 A}{\sin A} = \dfrac {\cos^2 A}{\sin A} \\ \sec A - \cos A = b^3 & \implies b^3 = \dfrac 1{\cos A} - \cos A = \dfrac {1-\cos^2 A}{\cos A} = \dfrac {\sin^2 A}{\cos A} \end{cases}

a 3 b 3 = cos 3 A sin 3 A a b = cos A sin A \begin{aligned} \implies \frac {a^3}{b^3} & = \frac {\cos^3 A}{\sin^3 A} \implies \frac ab = \frac {\cos A}{\sin A} \end{aligned}

a 3 b 3 = cos A sin A \begin{aligned} \implies a^3b^3 & = \cos A \sin A \end{aligned}

Then, we have:

cos A sin A + sin A cos A = cos 2 A + sin 2 A sin A cos A a b + b a = 1 a 3 b 3 a 2 + b 2 a b = 1 a 3 b 3 a 2 + b 2 = 1 a 2 b 2 \begin{aligned} \frac {\cos A}{\sin A} + \frac {\sin A}{\cos A} & = \frac {\cos^2 A + \sin^2 A}{\sin A \cos A} \\ \frac ab + \frac ba & = \frac 1{a^3b^3} \\ \frac {a^2+b^2}{ab} & = \frac 1{a^3b^3} \\ a^2+b^2 & = \frac 1{a^2b^2} \end{aligned}

a 2 b 2 = 1 a 2 + b 2 \quad \quad \implies \boxed{a^2b^2 = \dfrac 1{a^2+b^2}}

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