Question

The Sum of first n natural numbers is 210. What is n

Answer 1

Answer:

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Step-by-step explanation:

We have to find the sum n terms of the AP 1,2,3,...

Here

a=1,d=1.

Therefore, Required sum S

n

=

2

n

[2a+(n−1)d]=

2

n

[2×1+(n−1)×1]

⇒S

n

=

2

n

[2+n−1]

⇒S

n

=

2

n(n+1)

...(1)

Put n=20 in (1), we get

S

20

=

2

20(20+1)

=210