Question

If the common difference of an A.P. is 3, then find a20 – a15.​

Answer 1

Let the first term of the AP be a.

an = a(n − 1)d

a20 – a15 = [a + (20 – 1)d] – [a + (15 – 1)d]

= 19d – 14d

= 5d

= 5 × 3

Answer 2

Given: d=3

We know:

an=a+(n−1)d

∴a20−a15=[a+19d]−[a+14d]

                     =5d

                     =5×3

                     =15

Hence, the answer is 15.

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@Lk (Leaderboad king)