Question

21+24+27......+51 का मान क्या होगा

A.324
B.396
C.416
D.288​ ​

Answer 1

answer is 396 .hope it will help you

Answer 2

According to the Question

a = 21

l = 51 = Tn

d = 3

${\implies T_{n} = a + (n - 1)d}$

51 = 21 + (n - 1) × 3

Divides both sides by 3 we get

17 = n + 6

n = 17 - 6

n = 11

Now sum :-

${\implies S_{n} = \dfrac{n}{2}[2a + (n - 1)d]}$

${\implies S_{11} = \dfrac{11}{2}[2\times 21 + (11 - 1)3]}$

${\implies S_{11} = \dfrac{11}{2}[42 + 30]}$

${\implies S_{11} = \dfrac{11}{2}\times 72}$

= 11 × 36

= 396