求下列各式的值 已知cos(a-π/3)=12/13,π/3小于a小于π/2,求cosa

问题描述:

求下列各式的值 已知cos(a-π/3)=12/13,π/3小于a小于π/2,求cosa

π/3π/2-π/3>a-π/3>0
sin (a-π/3)=5/13
cosa=cos(a-π/3+π/3)=cos(a-π/3)cosπ/3-sin(a-π/3)sin(π/3)
=(12/13)(1/2)-(5/13)(√3/2)
=(12-5√3)/26

因为cos(a-π/3)=12/13,π/3