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