已知COS(A+π/4)=3/5,则SIN2A？

显示全部楼层 · 2012-12-11 23:57:31

COS(A+π/4)=3/5所以sin(A+π/4)=4/5,((COS(A+π/4))^2+（sin(A+π/4)）^2=1)由此可求得cosA=7√2/10,sinA=√2/10.所以SIN2A=2sinAcosA=7/25...

千问 · 2012-12-11 23:57:31

cos(A + π/4) = 3/5cosAcos(π/4) - sinAsin(π/4) = 3/5(1/√2)(cosA - sinA) = 3/5(cosA - sinA)2 = (3√2/5)2cos2A + sin2A - 2sinAcosA = 18/251 - sin2A = 18/2...