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Home Q & A Board Science-Related Homework Help Mathematics
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rjt6250 rjt6250
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12 years ago
>.< This is embarrassing, but how do you solve Quadratics by Factoring? (I'm a little stuck)
5k^2+43k-40=8k.
Please list the steps.

Thank you very much.
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mikejuiesmikejuies
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12 years ago
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12 years ago
5k^2+43k-40=8k.
You have to first get all the numbers on one side, so that they all equal 0.
So subtract 8k from each side:  5k^2+35k-40=0.
You know if you multiply 2 numbers together an the answer is 0, then one of them must equal 0, right? If you have ab = 0, then either a=0 or b=0.
So you have to separate  5k^2+35k-40 into two parts that multiply together: (.....)(.....)=0
You look at the  5k^2 and the 5 must be 5 x 1.
You look at the - 40, and there are lots o possible numbers that multiply to make 40. 4x10, 2x20, etc., and one of them has to be negative to make -40.
So you play around with the numbers a lot, guessing here and there, knowing that since the +35 is positive, then a bigger number will be multiplied by 5, and you end up with:
(5k   -5 )(1k + 8) =0

Now, either 5k -5 = 0, or 1k + 8 = 0,

Solve each of those for k and you'll get your two answers.
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