cos(a,b)=a*b/|a|*|b|=√2/2
a*b=√2/2*|a|*|b|=1
因为a+ λb与 λa-b的夹角是钝角
则
cos(a+λb,λa-b)=(a+λb)(λa-b)/|a+λb||λa-b|