Question

Solve this 1+2+3+_ _ _ _ _ _ _+21​

Answer 1

Step-by-step explanation:

here

t1=1. d=1 tn=21. n=?

by using formula ,

tn= a+(n-1)d

21= 1+(n-1)1

21=1+n-1

21=1-1+n

21 =n

n=21

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Answer 2

Given:

a ( First Term ) = 1

d ( common difference ) = 1

an ( Last Term ) = 21

To Find:

The sum of the first 21 natural numbers.

Formula:

an = a + ( n - 1 )d

S = n/2 ( a + an )

Solution:

an = a + ( n - 1 )d

21 = 1 + ( n - 1 )1

or, 21 - 1 = n - 1

or, 20 + 1 = n

or, 21 = n

S = n/2 ( a + an )

S = 21/2 ( 1 + 21 )

S = 21/2 ( 22 )

S = 21 × 11

S = 231

Answer:

The sum of the first 21 natural numbers is 231.