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Arithmetic Progression

If 18, a, b,-...

Question

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

(a) 19

(b) 7

(c) 11

(d) 15

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Solution

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,

Also,

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

Therefore,

Hence the correct option is (d).

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