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When I simplify $(3a-b^2-a)^3$, I used $(u±v)^3=u^3±3u^2v+3uv^2±v^3$

but I'm confused with $(b^2-a)$

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    You should slow down and abso$r$b the answe$r$s on your previous question before continuing to post questions of the same form.2012-07-14

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First of all, your cited identity is incorrect; the exponent on the $v$ was wrong in the last term. The correct version is $(u\pm v)^3=u^3\pm 3u^2v + 3uv^2\pm v^3.$ Also, note that $3a-b^2-a=2a-b^2$ so that $(3a-b^2-a)^3=(2a-b^2)^3,$ and you can then let $u=2a$ and $v=-b^2$, making the problem somewhat more straightforward.

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    I guest I need a tuto$r$ Thanks sir2012-07-14