Question

Find the sum of numbers between 49 to 500 divisible by 7?

Answer 1

solution :
1 st number divisible by 56
so 56+7.....+500

56, (56+7) 56+ (2*7) + (56+3*7) + .....+ (56 + 63*7)

Then the sum:
Sn = (n/2)*(an+a1)
= 64/2 (497+ 56)
= 32* 553
= 17,696