Question

设向量a=(1,2m),b=(m+1,1),c=(2,m),若(a+c)⊥b,则(a+b)·c=

Answer 1

(a+c)⊥b 得到ab+bc=0

m+1+2m+2（m+1）+m=0

解出m=-1/2

(a+b)·c=ac+bc=2+2m^2+2(m+1)+m=3