已知cos（八分之π-α）=五分之四，则sin（2α+四分之π）=
万分感谢！！
sin(2α+π/4)
=cos[π/2-(2α+π/4)]
=cos(π/4-2α)
=cos[2(π/8-α)]
=2cos²(π/8-α)-1
=2*(4/5)²-1
=7/25
一题是5/4=1.25
2*a=19.6
4/π=1.27
那么答案是19.6+1.27=20.87，sin【20.87】=0.356249
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万分感谢！！
sin(2α+π/4)
=cos[π/2-(2α+π/4)]
=cos(π/4-2α)
=cos[2(π/8-α)]
=2cos²(π/8-α)-1
=2*(4/5)²-1
=7/25
一题是5/4=1.25
2*a=19.6
4/π=1.27
那么答案是19.6+1.27=20.87，sin【20.87】=0.356249