已知sina=1/3,则sin(a+π/4)cos(a+π/4)
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sin(a+π/4)cos(a+π/4)
=(1/2)*sin[2(a+π/4)]
=(1/2)*sin(2a+π/2)
=(1/2)*cos(2a)
=(1/2)*[1-2*(sina)^2]
=(1/2)*[1-2*(1/3)^2]
=7/18
=(1/2)*sin[2(a+π/4)]
=(1/2)*sin(2a+π/2)
=(1/2)*cos(2a)
=(1/2)*[1-2*(sina)^2]
=(1/2)*[1-2*(1/3)^2]
=7/18
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