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solve x^2-18=7x by factoring

Sagot :

We subtract 7x from both sides so that the RHS would be 0.
x² - 7x - 18 = 0

We look for two numbers with a product of 18 and a difference of 7.
18 - 1 = 17  X
9 - 2 = 7  √
6 - 3 = 3  X 

So,
(x-9)(x+2)=0
This would mean that x=9 or x =-2 since one of thee factors need to be 0. (in order to know this, equate both factors to 0.)

Check:
If x = 9
9² - 7(9) - 18 = 0
81 - 63 - 18 = 0
18 - 18 = 0
0 = 0  This is indeed TRUE

If x = -2
(-2)² - 7(-2) -18 = 0
4 + 14 - 18 = 0
18 - 18 = 0
0 = 0  This indeed TRUE