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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)
