Question

A. Find (a) (b) (c)

B. (a) If ∑ n = 55 find n (b) If ∑ n = 210 find n.

C. Find the sum of all natural numbers below 50 which are divisible by 7?

Answer 1

(A) It is known that.

(B) It is known that.

a)

Since n is a natural number,

b)

Since n is a natural number,

(C) The list of natural numbers below 50 that are divisible by 7 is:

7, 14, 21, 28 … 49

It can be observed that these numbers form an A.P. whose first term is 7 and common difference is also 7.

∴ a = 7 and d = 7

Here,

The sum of all natural numbers below 50 that are divisible by 7 is 196.