Question

without adding find the sum of 1+2+3+4+5+...........+21​

Answer 1

Step-by-step explanation:

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Answer 2

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You Should First Know

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In A.P. the sum n terms are

$S_{n} = \frac{n}{2} (2a + (n - 1)d)$

or

$</em><em>S</em><em>_{n} = \frac{n}{2} (a + l)$

$\mathfrak{\huge{\purple {\underline{\underline{Solution}}}}}$

In the given question,

n=21

d=1

a=1

l=21

Use any of two formula,

$= > S_{n} = \frac{n}{2} (a + l) \\ = > \frac{21}{2} (1 + 21) \\ = > \frac{21}{\bf\cancel{2}} {\bf\cancel{22}} \\ = > 21 \times 11 \\ = > 231$