后面分母有理化
=(2√5+1)(√2-1+√3-√2+……+√20-√19)
=(√20+1)(√20-1)
=20-1
=19"分之1“是如何有理化的？1/(√2+1)=(√2-1)/[(√2+1)(√2-1)]=(√2-1)/(2-1)=√2-1其他同理