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# ​Factoring questions. I don't understand.

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Factoring questions. I don't understand.

Guest Sep 16, 2017
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3)   64j^15  + 125y^21

Notice that this is just the sum of two cubes....and we have

Remember that

a ^3 + b^3    =   ( a + b) (a^2 - ab + b^2)

So...let a  = 4j^5      b  = 5y^7     a^2  = 16j^10      b ^2 =  25y^14       and   -ab =  - 4j^5*5y^7

And we have.....

( 4j^5  + 5y^7) ( 16j^10 - 4j^5*5y^7 + 25y^14)  =

(4j^5  + 5y^7)  ( 16j^10  - 20 j^5y^7 + 25y^14)

4)  c^3d^3  + 125  =    (cd)^3  + 125

This is similar......

a ^3 + b^3    =   ( a + b) (a^2 - ab + b^2)

a  = cd     b  = 5      a^2   = (cd)^2       - ab  = - 5cd      and b^2 =  25

So we have

( cd + 5)  [ (cd)^2  - 5cd + 25]

CPhill  Sep 16, 2017

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