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Saturday, 14 November 2015

Quadratic Identity question

QUESTION:
Write 3x^2+7 in the form a(x-2)^2+b(x+3)+c.


ANSWER:
3x^2+7=a(x-2)^2+b(x+3)+c holds for all x.

Best way is to just expand the RHS and then equate coefficients of the powers of x on LHS and RHS.

3x^2+7=a(x^2-4x+4)+b(x+3)+c
3x^2+7=a x^2-4ax+4a+bx+3b+c
3x^2+7=a x^2+(b-4a)x+(4a+3b+c)

Therefore 
  1. a=3  (comparing coefficient of x^2 on LHS and RHS)
  2. b-4a=0 since coefft of x on LHS is 0. (there is no x term)
  3. 4a+3b+c=7 (Constant terms on LHS and RHS)
    So, b-4(3)=0.
    b=12.

    Finally, 4(3)+3(12)+c=7.
    So, c=7-12-36=-41.

    Final answer: a=3, b=12, c=-41

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