|a+tb|²=a²+2tab+t²b²
=5+8t+5t²
=5(t+4/5)²+9/5
当t=-4/5时|a+tb|²的最小值为9/5
故当t=-4/5时|a+tb|的最小值为3√5/5Ϊʲô�����tΪ-4/5t=-4/5ʱ��|a+tb|²=a²+2tab+t²b² =5+8t+5t² =5(t+4/5)²+9/5=0+9/5Ȼ����当t=-4/5时|a+tb|²的最小值为9/5故当t=-4/5时|a+tb|的最小值为3√5/5