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Numbers

Q1: The sum of first 45 natural numbers is:

A 1035

B 1280

C 2070

D 2140

ANS:A - 1035

Let Sn =(1 + 2 + 3 + ... + 45). This is an A.P. in which a =1, d =1, n = 45.

Sn = n [2a + (n - 1)d] = 45 x [2 x 1 + (45 - 1) x 1] = 45 x 46 = (45 x 23)
2 2 2
= 45 x (20 + 3) = 45 x 20 + 45 x 3 = 900 + 135 = 1035. Shorcut Method:
Sn = n(n + 1) = 45(45 + 1) = 1035.
2 2



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