Question

If 18, a, b, −3 are in A.P., the a + b =

Options

•  19

• 7

• 11

• 15

Answer 1

Here, we are given four terms which are in A.P.,

First term (a₁) = 18

Second term (a₂) =  a

Third term (a₃) =  b 

Fourth term (a₄)=  - 3 

So, in an A.P. the difference of two adjacent terms is always constant. So, we get,

 d =  a₂ - a₁ 

  d =  a - 18                              .................(1) 

Also,

 d = a₄ - a₃ 

 d = - 3 - b                           ...................(2) 

Now, on equating (1) and (2), we get,

  a - 18 = - 3 - b 

  a + b = 18 - 3

  a + b = 15 

Therefore,  a + b = 15