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Question
If the common differences of an A.P. is 3, then a20 − a15 is
Options
A. 5
B. 3
C. 15
D. 20
Solution
Let the first term of the A.P. be a.
`a_n=a(n-1)d`
`a_20-a_15=[a+(20-1)d]-[a+(15-1)d]`
`=19d-14b`
`=5d`
`=5xx3`
`=15`
The correct answer is C.
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