Find the slope of the line tangent to the curve f(x) = -10x^2 + 8x when x=9

The tangent line to f(x) at some point x has slope equal to f'(x), so first compute the derivative of f :

f(x) = -10x ² + 8x

f'(x) = -20x + 8

When x = 9, we have

f (9) = -10•9² + 8•9 = -738

f' (9) = -20•9 + 8 = -172

so the slope is -172.

If you also have to find the equation of the tangent line, use the point-slope formula:

y - (-738) = -172 (x - 9)

y = -172x + 810

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