a-b是第一象限角
sin(a-b)=√10/10
cos(a-b)=√100-10/10=3√10/10
b是第三象限角
tanb=1/2
sinb=-1/根号5=-根号5/5
cosb=-2/根号5=-2根号5/5
cosa=cos(a-b+b)
=cos(a-b)cosb-sin(a-b)sinb
=-6根号50/50+根号50/50
=-25根号2/50
=-根号2/2