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July 28, 2014

July 28, 2014

Posted by **superdiver** on Tuesday, October 2, 2012 at 6:01am.

If f(x)= (2x^2 - 5x - 3) / (3-x)

find f'(-8).

- calculus -
**Steve**, Tuesday, October 2, 2012 at 10:28amseparate the terms:

f(x) = 2x^2/(3-x) - 5x/(3-x) - 3/(3-x)

now use the quotient rule 3 times:

f'(x) = -2x(x-6)/(3-x)^2 - 15/(3-x)^2 - 3/(3-x)^2

However, this is a waste of time, since

(2x^2 - 5x - 3) = (x-3)(2x+1)

so, f(x) = -(2x+1)

f'(x) = -2 as long as x≠3

so, f'(-8) = -2

moral: see whether a function can be simplified before delving into long calculations.

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