已知π/2
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已知π/2已知π/2已知π/2由于π/2所以cos(π/4-a)cos(π/4+a)=cos(a-π/4)cos(π/4+a)=cos[(a-π/4)+(π/4+a)]-cos[(a-π/4)-(π/
已知π/2 已知π/2
已知π/2
由于π/2
cos(π/4-a)cos(π/4+a)=cos(a-π/4)cos(π/4+a)
=cos[(a-π/4)+(π/4+a)]-cos[(a-π/4)-(π/4+a)]
=cos2a-cosπ/2
=cos2a
=(cosa)^2 - (sina)^2
=1/8
又(cosa)^2 + (sina)^2 = 1
(cosa)^2=9/16,从而cosa=-3/4
(sina)^2=7/16,从而sina=√7/4
tana=-√7/3