First slide

Extreme values and Periodicity of Trigonometric functions

Question

$A + B = 90 ° \Rightarrow \cos A \cos B \in$

Easy

A

$[- \frac{1}{3}, \frac{1}{3}]$
B

$[- \frac{1}{2}, \frac{1}{2}]$
C

$[- \frac{1}{4}, \frac{1}{4}]$
D

$[- \frac{3}{4}, \frac{3}{4}]$

Solution

$A + B = 90 ° \Rightarrow \cos A \cos B$
$= \frac{1}{2} \sin 2 A \in [- \frac{1}{2}, \frac{1}{2}]$

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