Question

sum of 12 terms of the ap 9,17,25...​

Answer 1

Answer:

636

Step-by-step explanation:

The formula for the sum of the first n terms in an arithmetic progression is: Sn = n/2(2a + (n-1)d). n is the number of terms, a is the first number in the AP, and d is the difference between two numbers in the sequence one after another, so the value of (n+1) - the value of n.

d = 17 - 9 or 25 - 17 = 8

So plugging everything into the formula, S12 = 12/2(2 x 9 + (12 - 1)8) = 636

Not 100% sure though.

Also, remember the subscripts in the formula, I was not able to use subscripts here in the formula.

Answer 2

$\longrightarrow\large\sf\green{Given:-}$

$\longrightarrow\sf{a = 9}$

$\longrightarrow\sf{d = 8}$

$\longrightarrow\sf{n = 12}$

⠀

$\bigstar\large\sf\underline\bold\red{ To\: find \:the \:value \:of\: l (an)}$

$\longrightarrow\sf{an = a + (n - 1)d}$

$\longrightarrow\sf{an = 9 + (12 - 1) 8}$

$\longrightarrow\sf{an = 9 + (11) 8}$

$\longrightarrow\sf{an = 9 + 88}$

$\longrightarrow\sf{an = 97}$

⠀

$\bigstar\large\sf\underline\bold\blue{To \:find \:sum \:of \:first \:12\: terms}$

$\longrightarrow\sf{Sn = \dfrac{n}{2} (a + l )}$

$\longrightarrow\sf{Sn = \dfrac{12}{2} (9 + 97)}$

$\longrightarrow\sf{Sn = \dfrac{12}{2} (106) }$

$\longrightarrow\sf{Sn = \dfrac{\cancel{12}}{\cancel{2}} (106)}$

$\longrightarrow\sf{Sn = 6 \times 106}$

$\longrightarrow\sf{Sn = 636}$

$\longrightarrow\sf{So,the \:sum\: of\: first\: 12\: terms\: of\: the\: given\: ap\: is \:636 }$