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Find the critical values of P(x)

p(x)= (28+x)(8-x/4)

the answer was p=244-7x+8x-x^2/4

p=-x^2/4 + x + 244
p'= -x/2 +1 = 0
My question is, I do not understand how they got -7x + 8x at the beginning. I know (28)(8)=244 but I do not get the rest.

  • Math -

    Use FOIL:
    =28*8 -28*x/4 + 8*x -x*x/4
    =244 -7x + 8x -x²/4
    =244 + x -x²/4

    and the rest follows.

  • Math -

    Oh I see. I made a mistake with the 28 (x/4). Thanks!

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