If $18, a, b, - 3$ are in $A P$ then find $a + b$ .

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Solution

Let $p$ , be the first term i.e; $18$

And, $d$ , be the common difference,

From given series,

$4 t h$ term $= p + 3 d = - 3$

$⟹ 18 + 3 d = - 3$

Hence, $d = - 7$

So, $a = p + d = 18 - 7 = 11$

And, $b = p + 2 d = 18 - 14 = 4$

Therefore,

$a + b = 11 + 4 = 15$

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