Question

find the sum of numbers from 50to 120which are divisible by 7.

Answer 1

The numbers between 50 to 120 and are divisible be 7 will be in A.P ;
56,63,70............112,119

where ;
a = 56
d = 7

Tn = a+(n-1)d
119 = 56 + (n-1)7
63/7 = n-1
n = 10


Sum of 10 terms
= n/2 (a+l)
= 10/2 (56+119)
= 5 × 175
= 875

hence your answer is 875.
hope this helped you ...........!

Answer 2

no's divisible after50=56,63,,,,,,,,,,,,,119
=56+63+70+77+............+119
=7(8+9+10+........+17)
=7(1+2+3+4+.....+17)-7(1+2+3+4+.....+7)
sum of first n natural numbers is n(n+1)/2
=7(sum of first 17natural numbers)-7(sum of first seven natural numbers)
=7(17)(17+1)/2 - 7(7)(7+1)
=7(17)(18)/2-7(7)(8)/2
=7(17)(9)-7(7)(4)
=1071-196
=875