math

5 years from now a boy's age will be 2 time the square of what it was 5 years ago.how old is he now?
a)15/2 years c)7 years

b)3 years d) 8 years

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asked by rebika
  1. age now: x
    age in 5 years: x+5
    age 5 years ago: x-5

    so, write the words in math

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

    now just solve for x. Since you have a quadratic function, you will get two answers. Only one answer will make sense.

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    posted by Steve
  2. Now: X years old.
    5 yrs. ago: x-5 yrs. old.
    5 yrs. from now: x+5 years old.

    x+5 = 2*(x-5)^2.
    x+5 = 2(x^2-10x+25).
    x+5 = 2x^2-20x+50,
    -45 = 2x^2-21x,
    2x^2-21x+45 = 0.
    Use Quadratic Formula:
    X = (21 +- sqrt(441-360))/4 = 30/4 = 15/2, and 3.
    Use the larger number:
    X = 15/2 = 7.5 Years old.

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    posted by Henry

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