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

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

(a) 19

(b) 7

(c) 11

(d) 15


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

First term (a1) = 

Second term (a2) = 

Third term (a3) = 

Fourth term (a4)= 

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


$d=a-18$$\ldots \ldots$ (1)





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




Therefore, $a+b=15$

Hence the correct option is (d).


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