Solve by completing the square; round to the nearest hundredth 3×2+15×=108?

1 Answer
Sizadel
Mar 27, 2018

4, or -9

Explanation:

To solve the equation of #3x^2 + 15x = 108#, rearrange this first so that all the numbers are on the left,
#3x^2 + 15x-108# = 0
Then make the coefficient of #x^2# to 1. (Divide by 3)
That will be #x^2+5x-36#.
The formula for completing the square is
#(a+b/2)^2-(b/2)^2+c#.
So #(x+5/2)^2-25/4-36#
Next, simplify the constant (numbers without x)
#-36-25/4# is #-169/4#
Bring this number to the right and square root it to get rid of the square on the left-hand side.
#(x+5/2)=√169/4^#
Solve to make x the subject.
#x=-5/2+√169/4#
or it can be #x=-5/2-√169/4#

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