Find the sum of first 40 positive integers divisible by 6 also find the sum of first 20 positive integers divisible by 5 or 6.
Ans: No's which are divisible by 6 are
6, 12 ................ 240.
S_{40} = 40/2 [6 + 240]
= 20 x 246
= 4920
No's div by 5 or 6
30, 60 ............. 600
20/2 [30 + 600]= 10 x 630
= 6300