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Solve the quadratic equation by completing the square? (Algebra 2)

Equation: X^2 + 5X = 84

I am having a very hard time solving this, because I can't do fractions. Please help me step by step, and don't just say the answer. Thanks.

Respuesta :

Recall the (a^2+2ab+b^2) =(a+b)^2,
to complete the square for x^2+bx=c, add (b/2)^2 on both side, 
x^2+5x+(5/2)^2=84+(5/2)^2
(x+ 5/2)^2 =84+25/4
x+ 5/2 = √(84+25/4)
write 84 into 84*4/4, which is 336/4
336/4+25/4=361/4
so x +5/2=√(361)4)=19/2 or -19/2
x=19/2-5/2=14/2=7
or x=-19/2-5/2=-24/2=-12
so x is 7 or -12

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