Question

在三角形ABC中.角ABC所对应的边为abc.已知sinC/2=根号10/4求cosC

Answer 1

sin(C/2)=√10/4

由：[sin(C/2)]^2+[cos(C/2)]^2=1,解得

[cos(C/2)]^2=3/8

则：cosC=[cos(C/2)]^2-[sin(C/2)]^2=3/8-（√10/4）^2=-1/4