Question

In an AP of 21 terms if first term and last term are 4 and 64 respectively. then sum of first 21 terms ​

Answer 1

Answer:

714

Step-by-step explanation:

Let the AP be of the form,

a, a+d, a+2d,....., a+(r-1)d,......., a+20d.

Where a, d are real numbers.

General term i.e rth term of this AP is a+(r-1)d.

Given, a =4 (first term)

and a+20d=64 (last term).

⇒4+20d=64 ( since a=4)

⇒20d=60

⇒d=3.

We know that the sum of n terms of an AP is s= n/2 (2a + (n-1) d)

Therefore sum of first 21 terms is

s = 21/2 (2(4) + (21-1)3)

⇒s=21/2 (8+60)

⇒s=21 x 34

⇒s=714

Answer 2

Answer:

Sum of first 21 terms is 714.

Step-by-step explanation:

a = 4, l = 64, n = 21

Sum of first 21 terms = S$_{n}$

S$_{n}$ = n/2(a + l)

    = 21/2(4 + 64)

    = 21/2 x 68

    = 21 x 34

    = 714

I hope this answer is helpful for you.