Question

In an A.p if 6th&12th terms are 13&25 then find its 21th terms​

Answer 1

Answer:

tn = a + ( n - 1 ) x d

t6 = a + ( 6 - 1 ) x d

13 = a + 5d

i. e a + 5d = 13 ...( 1 )

tn = a + ( n - 1 ) x d

t12 = a + ( 12 - 1 ) x d

25 = a + 11d

i. e a + 11d = 25....( 2 )

Subtracting equation ( 1 ) from ( 2 )

a + 11d = 25

a + 5d = 13

6d = 12

d = 12 / 6

d = 2

Substituting d = 2 in equation ( 1 )

a + 5d = 13

a + 5 x 2 = 13

a + 10 = 13

a = 13 - 10

a = 3

tn = a + ( n - 1 ) x d

t21 = 3 + ( 21 - 1 ) x 2

= 3 + 20 x 2

= 3 + 40

= 43

t21 = 43

The 21 term of the A. P is 43.