sin(a b)=-3/5,sin(b-π/4）=12/13,求cos（a π/4)值網友廻答：
- cos（a π/4)
=cos[(a b)-(b-π/4)]
=cos(a b)cos(b-π/4) sin(a b)sin(b-π/4)
又sin(a b)=-3/5,sin(b-π/4）=12/13,
所以cos(a b)=4/5(a b爲第四象限角),cos(a b)=4/5(a b爲第四象限角)
cos(b-π/4)=5/13(b-π/4爲第一象限角),cos(b-π/4)=-5/13(b-π/4爲第二象限角),
所以cos（a π/4)=-16/65,-56/65
admin=-35,sin(b-π4）=1213,求cos（a+π4)值.jpg "sin(a+b)=-35,sin(b-π4）=1213,求cos（a+π4)值,第1張")
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