cos2A = cos^4 A - sin^4 A?

5 ⤊

Trig problem I need help with.
How can I prove this expression. Please help me!!

2007-11-28 15:57:13 · 2 answers · asked by anthony c 2 in Science & Mathematics ➔ Mathematics

RHS
cos²A cos²A - sin²A sin²A
cos^4A - (1 - cos²A)(1 - cos²A)
cos^4A - (1 - 2 cos²A + cos^4A)
2 cos²A - 1
cos 2A

LHS
cos 2A

LHS = RHS

2007-11-29 02:05:22 · answer #1 · answered by Como 7 · 3⤊ 1⤋

cos2A
=(cos^2 A - sin^2 A) x 1
=(cos^2 A - sin^2 A) x (cos^2 A + sin^2 A)
=cos^4 A - sin^4 A
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2007-11-29 00:06:14 · answer #2 · answered by thameera2007 2 · 2⤊ 2⤋