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March 28, 2015

Posted by **fareha** on Monday, December 5, 2011 at 5:32am.

a. Find the x-intercepts of the graph of f.

b. Find the x and y coordinates of all relative maximum points of f. Justify your answer.

c. Find the intervals on which the graph of f is increasing.

- calculus -
**Reiny**, Monday, December 5, 2011 at 8:38amMake a rough sketch of y = cos 2x

x-intercepts, when cos 2x = 0

2x = π/2 or 2x = 3π/2

x = π/4 or x = 3π/4

period of cos2x is π, thus adding/subracting π to any answer will yield a new answer.

π/4 - π = -3π/4

3π/4 - π = -π/4

x-intercepts are ±π/4, ±3π/4

f'(x) = 2sin 2x

= 0 for max/min

2sin 2x = 0

sin 2x= 0

2x = 0 , π , 2π

x = 0, π/2, π , again, since the period is π other answers are

x = -π/2, 0, -π

A quick look at your y = cos 2x will show you which of these are maximums and which are minimum

Max: at x = -π, 0, π

max points: (-π,2), (0,2), and (π,2)

To show where y = cos 2x is increasing,

f'(x) has to be positive

f'(x) = -2sin 2x has to be positive

so take a look where y = -2sin 2x lies above the x-axis

or

Since you have the sketch of y = cos 2x, it is easy to see that the curve is increasing between -π/2 and 0 and again between π/2 and π

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