Question

find the sum of first
15 terms of AP
2,7, 12....
please ans. the question fast​

Answer 1

Answer:

a. p=2, 7,12......

sn=n/2(2a+(n-1) d)

sn=15/2(2.2+(15-1) 5)

sn= 15/2(4+14(5))

sn= 7.5(4+70)

sn=7.5(74)

sn=555

Answer 2

• Sum of first 15 terms of A.P is 555 .

Step-by-step explanation:

To find:-

• Sum of first 15 terms of A.P .

Solution :-

Our A.P is

2, 7, 12 .....

First term = 2

Common difference:

$\sf \leadsto a_{2} - a_{1}$ {Where ,$\sf a_{2} \: is \: 7\: and \: a_{1} \: is \: 2$}

$\sf \leadsto 7 - 2$

$\sf \leadsto 5$

We know

$\boxed{ \sf \bold{ S_{n} = \dfrac{n}{2} \big\{2a + (n - 1)d \big\} }}$

• Where, n is number of terms, a is first term and d is common difference of A.P .

Put, n, a and d in formula:

$\sf \longrightarrow S_{15} = \dfrac{15}{2} \big\{2 \times 2 + (15 - 1) \times 5 \big \}$

$\sf \longrightarrow S_{15} = \dfrac{15}{2} \big\{4 + (14) \times 5 \big\}$

$\sf \longrightarrow S_{15} = \dfrac{15}{2} \big\{ 4 + 70 \big\}$

$\sf \longrightarrow S_{15} = \dfrac{60}{2} + \dfrac{1050}{2}$

$\sf \longrightarrow S_{15} = \dfrac{1110}{2}$

$\sf \longrightarrow \blue{ \boxed{ \sf \bold{S_{15} = 555}} \bigstar}$

Therefore,

Sum of first 15 terms of A.P is 555.