Question

which term of A.P. 3,7,11,15,...... is 79? Also find the sum 3+7+11+....+79​

Answer 1

Answer:

13 th term is the answer

Answer 2

Answer:

The 20th term is 79

3 + 7 + 11 + 15 +...........+ 79 = 820

Step-by-step explanation:

We are given,

A.P = 3, 7, 11, 15,........

Thus,

a = 3

d = a2 - a1 = 7 - 3 = 4

Now,

Let nth term be 79

So,

a = 3

a(nth) = 79

d = 4

So,

79 = 3 + (n - 1)4

79 - 3 = (n - 1)4

(n - 1)4 = 76

n - 1 = 76/4

n - 1 = 19

n = 19 + 1

n = 20

Thus,

the 20th term is 79

Now,

Sum of n terms = (n/2)[a + l]

where l is the last term

Here,

n = 20

a = 3

l = 79

S(20) = (20/2)[3 + 79]

S(20) = 10 × 82

S(20) = 820

Hence,

3 + 7 + 11 + 15 +...........+ 79 = 820

Hope it helped and believing you understood it........All the best