cosa=√5/5则cos(a-π/4)=?
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cosa=√5/5则cos(a-π/4)=?
cosa=√5/5则cos(a-π/4)=?
cosa=√5/5则cos(a-π/4)=?
cosa=√5/5
sina=±2√5/5
当sina=2√5/5时
cos(a-π/4)
= cosacosπ/4+sinasinπ/4
=√5/5*√2/2+2√5/5*√2/2
=√10/10+2√10/10
=3√10/10
当sina=-2√5/5时
cos(a-π/4)
= cosacosπ/4+sinasinπ/4
=√5/5*√2/2-2√5/5*√2/2
=√10/10-2√10/10
=-√10/10
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