Question

Find the sum of all natural numbers lying between 100 and 700 which are multiples of 7

Answer 1

Answer:

Ans is 33915.

Step-by-step explanation:

105,112...693. a= 105, d= 7 n=? An= 693

An = a+(n-1)d.

693 = 105+(n-1)7

693-105 = (n-1)7

588/7 = n-1

84+1 = n

n = 85

Sn = n/2 [2a+(n-1)d]

= 85/2 (2*105+85-1*7)

= 85/2 (210+588)

= 85/2 (798)

= 85*399

= 33915