Question

请问已知cosa=3/5,a属于(0,π/2),则cos(a-π/4)=?

Answer 1

cosa=3/5,a属于(0,π/2)可得：
sina=√(1-cos^2a)=4/5

cos(a-π/4)
=cosacosπ/4+sinasinπ/4
=3√2/10+4√2/10
=7√2/10

Answer 2

cosa=3/5,a属于(0,π/2)
所以
sina=4/5
所以
cos(a-π/4)=cosacosπ/4+sinasinπ/4=√2/2(3/5+4/5)=7√2/10

Answer 3

由条件得：sina = 4/5
cos(a-π/4)= cosa * cos(π/4) + sina * sin(π/4) = 3/5 * (二分之根二) + 4/5 * (二分之根二) = (7*根2/10)

Answer 4

因为cosa=3/5 ， a属于(0,π/2), 所以 sina=4/5
cos(a-π/4)=cosacosπ/4+sinasinπ/4=3/5*√2/2 +4/5*2/2=7√2/10