Question

If the 3rd and the 9th terms of an A.P. are 4 and - 8 respectively, then which term of
this A.P is -10.
A.12
B.9
C.10
D. 11​

Answer 1

3rd term= 4

9th term = -8

t n = a+(n-1)d

Remember whenever such a question comes we will get 2 equations

4= a +2d ( mark this 1)

-8 = a+8d ( mark this 2)

subtracting 1 from 2

a+8d = -8

-a-2d =-4

= 6d = -12

d= -2

putting d value in 1,

4 = a +2*-2

4 = a-4

a= 8

Now we need to find which term of ap is -10

-10 = 8+(n-1)-2

-10-8 = -2n +2

-18 -2 = -2n

-20 = -2n

n = 19

correct option c