Question

设sina:sin(a/2)=8:5,则cosa=

Answer 1

sina:sin(a/2)=8:5,

2sin(a/2)cos(a/2):sin(a/2)=8:5,

2cos(a/2)=8:5

cos(a/2)=4/5

则cosa=2[cos(a/2)]^2-1

=2(16/25)-1

=32/25-1

=7/25