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Posted by on Friday, December 31, 2010 at 4:08am.

I have a number of problems I'm having problem with but they start with the example given in the book

2x^2 - 10x
=2(x^2 - 5x)
=2{[x-(5/2)]^2 - [(5/2)^2]}
=2[x-(5/2)]^2 - (25/2)

Why isn't (5/2)^2 (25/4)?

I'm struggling with the exercises as well.

3x^2 - 24x
I get 3(x-4)^2 - 16 or if I try to follow the example, 3(x-4)^2 - 32, but the answer at the back of the book is 3(x-4)^2 - 48

  • Math; Complete the square for the expressions - , Friday, December 31, 2010 at 4:10am

    I meant, "Why isn't (25/2)^2 (25/4)?"

  • Math; Complete the square for the expressions - , Friday, December 31, 2010 at 7:09am

    You seem to have forgotten a basic rule of multiplying fractions.

    a/b * c/d = ac/(bd)

    The square of any fraction a/b is a^2/b^2.

    For example, 2/3 of 2/3 = (2/3)^2 = 4/9
    and (25/2)^2 = 625/4

    Looking at your second problem,
    3x^2 - 24x
    = 3(x^2 -8x) Now add and subtract 48
    = 3(x^2 -8x +16) -48
    = 3(x-4)^2 -48

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