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问题: 救命!!!二倍角

已知0<a<π/4,sina=4/5
求tan(a-5π/4)的值

解答:

sina=4/5, cosa=3/5, tana=4/3
tan(a-5π/4) =[tana-tan(5π/4)]/[1+tana*tan(5π/4)] =1/7