Question

which term of ap 5,13,21 will be 72 less than its 17 term

Answer 1

let , first term a = 5
commom difference (d ) = 13 - 5 = 8
d = 8
T 17 = a + 16d

T17 = 5 + 16 ( 8 )

T17 = 5 + 128

T17 = 133

required term

T17 - 72 = a + ( n - 1 ) d

133 - 72 = a + ( n - 1 )d

61 = 5 + ( n - 1 )8

61 - 5 = 8n - 8

56 + 8 = 8n

8n = 64

n = 8

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your answer : T 8
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