sin^4 (5π/8)-cos^4 (5π/8) 求值
题目
sin^4 (5π/8)-cos^4 (5π/8) 求值
答案
sin^4 (5π/8)-cos^4 (5π/8)
=〔sin^2 (5π/8)-cos^2(5π/8)〕〔sin^2 (5π/8)+cos^2 (5π/8)〕
=sin^2 (5π/8)-cos^2(5π/8)
=1-2cos^2(5π/8)
=-cos(2*5π/8)
=-cos(5π/4)
=√2/2
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