Question

3+5+7+9+....+21=? please solve this​

Answer 1

Answer:   120

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Step-by-step explanation:

3+5+7+9+..........+21 = ?

This is an A.P.

First term = a = 3 , last term = $l$ = 21 ,

difference of two consecutive terms = d = $a_{2} -a_{1}$ = 5 - 3 => d = 3

number of terms = n => $l =a+(n-1)d$

                                  => 21 = 3 + ( n- 1 )2

                                  =>  18 = ( n - 1 )2

                                  =>   9 + 1 = n

                                  =>   n = 10

we know that sum of " n " terms is :  $S_{n} =\frac{n}{2} (a+l)$

=>  $S_{n}=\frac{10}{2} (3+21)$

=>   $S_{n}=5(24)$

=>   $S_{n}$ = 120