Question

- If 21, a, b and -3 are in A.P., then find the value of (a + b).
find the value​

Answer 1

Step-by-step explanation:

a=21

T4 (4th term) = -3

T4 = a + (n-1)d

-3 = 21 + (4-1)d

-3 -21 = 3d

-24 = 3d

d = -8

now you can calculate it directly by subtracting 8 in each term but incase you want to use formula

T2 = 21 + (1)-8

= 21-8

= 13

T3= 21 + (2)-8

= 21-16

= 5

a=13 and b=5

so value of a+b is 18