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HELP

// h (red) approximates the derivative of f (blue)

function f(x) { y = Math.cos(x); return y }
function h(x) { dx=(bX-aX)/1e5; y = (f(x+dx)-f(x-dx))/(dx*2); return y }

aX = -10; bX = 10; aY = -1.5; bY = 1.5
Dx = 1; Dy = 0.5