SOLUTION: 3X2 - 21X + 36=0

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Question 74875: 3X2 - 21X + 36=0
Answer by bucky(2189) About Me  (Show Source):
You can put this solution on YOUR website!
3x%5E2+-+21x+%2B+36=0
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First notice that 3 is a common factor of all three terms on the left side. Factor it
out and the left side becomes:
.
3%2A%28x%5E2+-+7x+%2B+12%29+=+0
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Now divide both sides of the equation by 3 and the result is:
.
x%5E2+-+7x+%2B+12%29+=+0
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Since the coefficient (multiplier) of the x%5E2 term is 1, you can ask yourself,
"What are the factors of 12 that might add together to give me the -7 on the middle term?"
(Notice that the factors of 12 have to either both be positive or both be negative
since they need to multiply together to give +12.)
.
How about -3 and -4? They will multiply together to give you +12 and they will add together
to give you -7.
.
That works. Therefore, the two factors are (x - 3) and (x - 4). If you multiply them
the result will be x%5E2+-+7x+%2B+12 and that is what we were looking for.
.
So now the problem is reduced to:
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%28x-3%29%2A%28x%2B4%29+=+0
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This equation will be true if either of the factors is zero. Therefore, you know that either:
.
%28x-3%29+=+0
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or
.
%28x+-4%29=0
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If you solve these two equations, you will get two answers ... x = 3 and x = 4. These are
the two values of x that will make your original equation true ... substitute 3 for x
and see if it doesn't make the left side of the original equation equal zero. Then
substitute 4 for x and see if that also doesn't make the left side of the original
equation equal zero.
.
Hope this helps you to see how you can solve this equation by applying some basic rules
of algebra.