Question

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

Answer 1

Answer is given in the pic.

And don't forget to solve my question.

Answer 2

21, a, b and -3 is the given series

then,first term(A)=21

common difference (D)=D

then,

second term(a_2):A+D=a

third term(a_3):A+2D=b

fourth term(a_4):A+3D=-3

next add second and third terms we get,

A+A+D+2D=a+b

A+A+3D=a+b

from fourth term A+3D=-3

and first term A=21

21-3=a+b

18=a+b

so the value of a+b is 18