Question

Find S10 for 2 + 5 + 8 +...

Answer 1

here n=10, d=3, a=2

Answer 2

answer is 155...

Answer 3

Here first term (a) is : 2
Here common difference is : 5 - 2 = 3

So:

\[a_n = a + (n - 1) d\]

Here, n = 10

\[a_{10} = 2 + (10 - 1) (3)\]
\[a_{10} = 29\]

Now use :

\[S_n = \frac{n}{2}[a + l]\]

here: \(a_{10} = l = 29\)

Answer 4

Here n = 10:

So just put n = 10 in the above formula for sum and then calculate, you will definitely get your answer..

Answer 5

@waterineyes u dont need to calculate last term first... just apply the sum to n terms formula...

Answer 6

And there is no harm if you calculate the last term..

Answer 7

And in your formula too you are calculating the last term but indirectly..
Look for the colored part here:

\[S_n = \frac{n}{2}[2a + (n-1)d] \implies S_n = \frac{n}{2}[a + \color{blue}{a + (n - 1)d}]\]