Paul H. answered • 01/01/18

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Derivative of cos(x) is (-1 times sin(x))

Since sin(0) = 0, the derivative of cos at x=0 is also 0 (zero can be neither positive nor negative; it is unsigned)

You can also look at the graph of y=cos(x) and see that at x=0, the tangent line is flat (parallel to the x-axis) which means that the slope of the curve is zero

Mark M.

01/01/18