Question

Find the sum of all numbers between 100 and 1000 which are divisible by 11.

Answer 1

The 1st greatest No. greater than 100 is 110. the last which is less than 1000 is, 990.

110,121, ………990 be the number

Tn = 90 ⇒ 110 + (n – 1) 11 = 990

11n – 11 = 990 – 110

11n = 880 + 11 = 891

∴ n = 801/1 = 81

Now S81 = 8(1/2) (110 + 990) = 81/2(550) = 4450