在Rt△ABC中,角C=90°,求证sin^2A+sin^2B=1,并利用上式求sin^21°+sin^22°+sin^32°+……+sin^289°的值

问题描述:

在Rt△ABC中,角C=90°,求证sin^2A+sin^2B=1,并利用上式求sin^21°+sin^22°+sin^32°+……+sin^289°的值

因为sin^2A+cos^2A=1
cos2A=cos^2A-sin^2A
所以cos^2A=(cos2A+1)/2
sin^2A=(1-cos2A)/2
因为A+B=90°
所以2A+2B=180°
所以cos2B=COS(180°-2A)=-COS2A
所以cos2B+cos2A=0
所以sin^2A+sin^2B==(1-cos2A)/2+(1-cos2B)/2=1