Question

Find the sum of last 20 terms of an A.S.(Arithmetic Sequence) 3,7,11,.........407

Plz answer it fast. It's urgent

Answer 1

Answer:

Step-by-step explanation:

Answer 2

The sum of 20 terms of A.P is 1424.5

Step-by-step explanation:

Given as :

For Arithmetic Progression

The A,P series are  3 , 7 , 11 , .............. , 407

Let The sum of 20 terms = $S$

From the series ,

The first term = a = 3

Common difference = d = 7 - 3 = 4

Now,

∵ The sum of n term of an A.P = $S_n$

And   $S_n$  = $\dfrac{n}{2}$  × [ first term + last term ]

For  n = 407

Or,   S   = 203.5  × [ a  +  d ]

Or,   S  = 203.5  × [ 3  +  4 ]

Or,   S  =  203.5 × 7

∴      S = 1424.5

So, The sum of 20 terms of A.P =    = 1424.5

Hence, The sum of 20 terms of A.P is 1424.5   Answer