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AP® Calculus AB-BC

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Moderate

Horizontal Tangent Line

APCALC-BSGISJ

How many horizontal tangent lines exist for the following function?

$$f( x ) =\cos { x-\cfrac { 1 }{ 2 } x\quad \text{on}\quad [ 0,2\pi ] } $$

A

$3$

B

$2$

C

$1$

D

$0$