`**) (b - a)/(a. b) = 1/a - 1/b`
Giải
`x/110 + x/132 + x/156 + ... + x/9900 = 3 3/5`
`⇒ x xx (1/110 + 1/132 + 1/156 + ... + 1/9900) = 18/5`
`⇒ x xx (1/(10 xx 11) + 1/(11xx12) + 1/(12 xx 13) + ... + 1/(99 xx 100) ) = 18/5`
`⇒ x xx (1/10 - 1/11 + 1/11 - 1/12 + 1/12 - 1/13 + ... + 1/99 - 1/100) = 18/5`
`⇒ x xx (1/10 - 1/100) = 18/5`
`⇒ x xx 9/100 = 18/5`
`⇒ x = 18/5 : 9/100`
`⇒ x = 40`
Vậy `x = 40`
