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प्रश्न
Find the number of terms in the following A.P.
`18,15 1/2, 13`, ..., – 47
How many terms are there in the AP?
`18, 15 1/2, 13`, ..., -47
उत्तर
For this A.P.
a = 18
`d = a_2-a_1=15 1/2-18`
`d = (31-36)/2 = -5/2`
Let there are n terms in this A.P.
Therefore, an = −47 and we know that,
an = a + (n − 1) d
-47 = `18 + (n - 1) (-5/2)`
-47 - 18 = `(n - 1) (-5/2)`
-65 = `(n - 1)(-5/2)`
(n - 1) = `- 65xx (-2/5)`
(n - 1) = 26
n = 26 + 1
n = 27
Thus, the required number of terms is 27.
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