Annoying Trig

Algebra Level 3

Given that x R x \in \mathbb R , find the minimum value of

( 3 5 4 cos x + 13 12 sin x ) 2 \left(3 \sqrt{5-4 \cos x}+ \sqrt{13-12 \sin x}\right)^2

Hint: It's on the draft paper of a geometry problem.


The answer is 40.

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1 solution

Let us draw a triangle A B C \triangle {ABC} with A B = 3 , A C = 6 , B A C = x |\overline {AB}|=3, |\overline {AC}|=6, \angle {BAC}=x . Then B C = 3 5 4 cos x |\overline {BC}|=3\sqrt {5-4\cos x} . Let us draw a line A D \overline {AD} perpendicular to A C \overline {AC} such that A D = 2 |\overline {AD}|=2 . Join B D \overline {BD} . Then B D = 13 12 sin x |\overline {BD}|=\sqrt {13-12\sin x} . The minimum of B D + B C |\overline {BD}|+|\overline {BC}| is C D = 6 2 + 2 2 = 40 |\overline {CD}|=\sqrt {6^2+2^2}=\sqrt {40} , so that the required minimum is 40 \boxed {40} .

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