1 已知tana=7/24,求cos2A的值
2已知sinB乘cosB=60/169,且π/4<B<π/2,求sinB个cosB的值
tanA=sinA/cosA=7/24 24sinA=cos7
sin^A+cos^A=1
49cos^A/576+cos^A=1
cos^A=576/625
cos2A=2cos^A-1=2x576/625-1=527