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 February 28th, 2016, 01:38 PM #1 Newbie   Joined: Feb 2016 From: washington Posts: 4 Thanks: 0 Prove Question I can't seem to get the answer but I think I'm close. The question is sin^4(x) - cos^4(x) = 1 - 2cos^2(x) I got sin^2(x)-cos^2(x)/sin^2(x)+cos^2(x), trying to prove LHS. Thanks !
 February 28th, 2016, 01:50 PM #2 Math Team     Joined: Jul 2011 From: Texas Posts: 2,945 Thanks: 1552 $\sin^4{x}-\cos^4{x}=$ $(\sin^2{x}-\cos^2{x})(\sin^2{x}+\cos^2{x})=$ $(\sin^2{x}-\cos^2{x}) \color{red}{(1)}=$ $(1-\cos^2{x})-\cos^2{x}=$ $1-2\cos^2{x}$ Thanks from v8archie and Country Boy Last edited by skeeter; February 28th, 2016 at 02:08 PM.

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