Find the equation of the line tangent to the graph of y = cos(2x) at x = pi/4

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y=cos(2x) 
dy/dx=-2sin(2x) 
dy/dx at the point x=pi/4 is 
dy/dx=-2sin(2*pi/4) 
dy/dx=-2 sin(pi/2) 
dy/dx=-2 
therefore the slope of the tangent is -2 

using this slope find the equation of the line 
(y-y')=m(x-x') 
(y-0)=-2(x-pi/4) 
y=-2(x-pi/4)

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