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If the Common Differences of an A.P. is 3, Then A20 − A15 is - Mathematics

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ConceptSum of First n Terms of an AP

Question

If the common differences of an A.P. is 3, then a20 − a15 is

• 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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Solution If the Common Differences of an A.P. is 3, Then A20 − A15 is Concept: Sum of First n Terms of an AP.
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