1-√2/(1-√2)(1+√2)+ (√2-√3)/(√2-√3)(√2+√3)+....+(√8-√9)/(√8+√9)(√8-√9)=-(1-√2)- (√2-√3)-(√3-√4)-....-(√8-√9)=-1+√9 =2
∵1／[√n＋√﹙n＋1﹚]=[√﹙n＋1﹚＋√n][√﹙n＋1﹚-√n]／[√n＋√﹙n＋1﹚]＝√﹙n＋1﹚－√n∴√2－1＋√3－√2＋。。。＋√9－√8＝√9－1＝2
