# Perfect cube decomposition question

• Nov 28th 2009, 11:27 AM
vd853
Perfect cube decomposition question
can someone explain to me how they got $(4x - y)$ in the numerator?

problem: $( 64x^3 - y^3 ) / ( 4x^2 - 33xy + 8y^2 )$

first step: $[ ( 4x - y ) ( 16x^2 + 4xy + y^2 ) ] / [ ( 4x - y ) ( x - 8y ) ]$

I know they used the perfect cube formula, but why use 16 and 4. and where did the $(4x - y)$ come from?
• Nov 28th 2009, 11:54 AM
masters
Quote:

Originally Posted by vd853
can someone explain to me how they got $(4x - y)$ in the numerator?

problem: $( 64x^3 - y^3 ) / ( 4x^2 - 33xy + 8y^2 )$

first step: $[ ( 4x - y ) ( 16x^2 + 4xy + y^2 ) ] / [ ( 4x - y ) ( x - 8y ) ]$

I know they used the perfect cube formula, but why use 16 and 4. and where did the $(4x - y)$ come from?

Hi vd853,

The general factorization model for the differernce of two cubes is:

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

Your numerator could be re-written as $((4x)^3-y^3)$

Using this and the model, we come up with $(4x-y)(16x^2+4xy+y^2)$
• Nov 28th 2009, 02:55 PM
vd853
Thanks! did not know about the difference formula.