SOLUTION: four times the square of a number is 21 more than eight times the number. what is the number?

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Question 422920: four times the square of a number is 21 more than eight times the number. what is the number?
Answer by ewatrrr(24785) About Me  (Show Source):
You can put this solution on YOUR website!


Hi

Let x represent the number

Question states***Write as we Read

 4x^2 = 8x + 21

Solving for x

  4x^2 - 8x - 21 = 0

 x+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+ 

 x+=+%288+%2B-+sqrt%28+400+%29%29%2F%288%29+  

    x = 1%2B-+20%2F8

    x  = 28/8 = 7/2

or x = -12/8 = - 3/2