SOLUTION: Complete the steps in solving the quadratic function 7x – 9 = 7x2 – 49x by completing the square.

Algebra ->  Quadratic Equations and Parabolas -> SOLUTION: Complete the steps in solving the quadratic function 7x – 9 = 7x2 – 49x by completing the square.       Log On


   



Question 1054500: Complete the steps in solving the quadratic function 7x – 9 = 7x2 – 49x by completing the square.

Found 2 solutions by Fombitz, Edwin McCravy:
Answer by Fombitz(32388) About Me  (Show Source):
Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!
7x+-+9%22%22=%22%227x%5E2+-+49x
Swap sides since the squared term in on the right:

7x%5E2+-+49x%22%22=%22%227x+-+9

Get all terms in x on the left and the constant 
term on the right:

7x%5E2+-+56x%22%22=%22%22-9

Divide every term by the coefficient of x², which is 7

expr%287%2F7%29x%5E2+-+expr%2856%2F7%29x%22%22=%22%22expr%28%28-9%29%2F7%29

x%5E2+-+8x%22%22=%22%22-9%2F7

To the side, take the coefficient of x, which is -8,
multiply it by 1%2F2, -8%2Aexpr%281%2F2%29=-4.
Now square the -4, getting %28-4%29%5E2=%22%22+%2B+16.
Add +16 to both sides of the equation:

x%5E2+-+8x%2B16%22%22=%22%22-9%2F7%2B16

Factor the left side.
Get an LCD and add the fractions on the right:

%28x-4%29%28x-4%29%22%22=%22%22-9%2F7%2B16%2Aexpr%287%2F7%29

The left side has two equal factors, so we write
it as the square of one of the factors. Work some
more on the right side:

%28x-4%29%5E2%22%22=%22%22-9%2F7%2B112%2F7%29

Finish the right side:

%28x-4%29%5E2%22%22=%22%22103%2F7%29

We use the principle of square roots:


x-4%22%22=%22%22%22%22+%2B-+sqrt%28103%2F7%29

We rationalize the denominator on the right:

x-4%22%22=%22%22%22%22+%2B-+sqrt%28expr%28103%2F7%29expr%287%2F7%29%29

x-4%22%22=%22%22%22%22+%2B-+sqrt%28721%2F49%29

x-4%22%22=%22%22%22%22+%2B-+sqrt%28721%29%2F7

x%22%22=%22%224+%2B-+sqrt%28721%29%2F7

x%22%22=%22%224%2Aexpr%287%2F7%29+%2B-+sqrt%28721%29%2F7


x%22%22=%22%2228%2F7+%2B-+sqrt%28721%29%2F7

x%22%22=%22%22%2828+%2B-+sqrt%28721%29%29%2F7

Edwin