The sum of a whole number and twice its square is 21. find the whole number

SOLUTION:

Let whole number = x

x + 2x^2 = 21

2x^2 + x - 21 = 0

2x^2 + 7x - 6x - 21 = 0

x ( 2x + 7 ) - 3 ( 2x + 7 ) = 0

( 2x + 7 ) ( x - 3 ) = 0

2x + 7 = 0

2x = - 7

x = - 7 / 2

OR

x - 3 = 0

x = 3

ANSWER:

Therefore, as 3 is a whole number while - 7 / 2 isn't a whole number, the whole number must be 3.

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