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Question
If 18, a, b, −3 are in A.P., the a + b =
Options
19
7
11
15
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Solution
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
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