Question

The 4th and 7th term of an AP. are 17and 23repectively find a15

Answer 1

Given, 4 th and 7th term of an A.P is 17 and 23.

since , we know

an = a + ( n - 1 ) d

a4 = a + ( 4 - 1 ) d = a + 3d.

a7= a + ( 7 - 1 ) d = a + 6d

according to question,

a4 = 17

a + 3d = 17 --- (1)

a7 = 23

a + 6d = 23 -- (2)

subtract .(1) and (2) , we get

a + 3d = 17

a + 6d = 23
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-3d = - 6 => d = 2

put value of 'd' in eq. (1) , we get

a + 3( 2 ) = 17

a = 17 - 6 = 11

therefore,

a15 = a + ( 15 - 1 )d = a + 14d

a15 = 11 + 14 (2 ) = 11 + 28 =39

a15 = 39

Your Answer : a15 = 39
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