∴⊥,即∠ABC=90°.
【例5】设e1,e2是夹角为45°的两个单位向量,且a=e1+2e2,b=2e1+e2,求|a+b|的值.
解:因a+b=3e1+3e2,
所以|a+b|2=|3e1+3e2|2=9(e1+e2)2
=9(e12+2e1e2+e22)
=9(1+2×1×1×cos45°+1)
=9(2+),
∴|a+b|=.