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13) Suppose that the function f is defined by f(x) = 5x3 -10x2-8x + 25 , with domain [-4, 8]. a) Calculate the slope of the secant line that connects the two points (-4, f(-4) ) & (8, f(8) ). This slope is b) Explain how we can be certain that there exists a number k between -4 and 8 such that the first derivative of f evaluated at k equals 192