-1/2+cos^2（7/8π）
=1/2*[2cos^2（7/8π）-1]
=1/2*cos(7/4π）
=1/2*cos(1/4π)
=√2/4
2sin^2（3/4π）-1
=-[1-2sin^2(3/4π)]
=-cos(3/2π)
=0
计算：-1/2+cos^2（7/8π） 2sin^2（3/4π）-1
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