已知sin(a-π/4)=-根号2/10,0

问题描述:

已知sin(a-π/4)=-根号2/10,0

数学人气:813 ℃时间:2020-03-17 17:40:55
优质解答
由a范围
则cos(a-π/4)>0
sin²+cos²=1
所以cos(a-π/4)=7√2/10
cosa=cos(a-π/4+π/4)
=cos(a-π/4)cosπ/4-sin(a-π/4)sinπ/4
=4/5
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由a范围
则cos(a-π/4)>0
sin²+cos²=1
所以cos(a-π/4)=7√2/10
cosa=cos(a-π/4+π/4)
=cos(a-π/4)cosπ/4-sin(a-π/4)sinπ/4
=4/5