Question

Find the sun of all natural number between 100 and 500 which are divisible by 7

Answer 1

105+112+119+...…...+497
105+(n-1)7=497
105+7n-7=497
7n=399
n=57
sum=n(a+l)/2
sum=57(105+497)/2
sum=
15351

Answer 2

First number = 105 = a
Last number = 497 = l

difference = 7

××××××××××××××××

l = a + (n - 1)d
497 = 105 + (n - 1)7
497 - 105 = (n - 1)7
392 = (n-1)7
56 = n - 1
57 = n


Sum = (105+497) × 57/2

=> (602)×57/2

=> 301 × 57

=> 17157


I hope this will help you

(-: