Question

39.9X2.5简便计算

Answer 1

39.9X2.5
=（40-0.1）×2.5
=100-0.25
=99.75

Answer 2

39.9X2.5 =（40-0.1）×2.5 =100-0.25 =99.75

Answer 3

原式=（40-0.1）×2.5
=40×2.5-0.1×2.5
=100-0.25
=99.75

追答

请采纳

Answer 4

k

Answer 5

考察一般项第k项： (2k)2/[(2k-1)(2k+1)] =4k2/(4k2-1) =(4k2-1+1)/(4k2-1) =1 +1/(4k2-1) =1+ 1/[(2k-1)(2k+1)] =1+ ?[1/(2k-1) -1/(2k+1)] 22/(1×3)+ 42/(3×5) +62/(5×7)+ 82/(7×9) +102/(9×11) +122/(11×13) =6+?(1- 1/3 +1/3- 1/5+ 1/5-1/7 +1/7-1/9 +1/9-1/11+ 1/11-1/13) =6+?(1- 1/13) =84/13 一般的： 22/(1×3)+ 42/(3×5) +...+(2n)2/[(2n-1)(2n+1)] =n+?[1- 1/3 +1/3- 1/5+...+1/(2n-1) -1/(2n+1)] =n+?[1- 1/(2n+1)] =n+ n/(2n+1) =2n(n+1)/(2n+1) 本题项数很少，硬算也是可以的，但是，当项数很多时，靠硬算就不行了。因此把详细的推导过程写出，供你参考。