If the common difference of an A.P. is 3, then a20 – a15 is (CBSE 2011)

We know:an = a + (n – 1)dSo,a20 – a15 = a + (20 -1 ) d – ( a + (15 – 1)d)= a + 19 d – a – 14d= 5 d= 5× 3= 15

Q1 of 84 Page 5

If the common difference of an A.P. is 3, then a₂₀ – a₁₅ is (CBSE 2011)

We know:

a_n = a + (n – 1)d

So,

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

= a + 19 d – a – 14d

= 5 d

= 5× 3

= 15

In an AP, if d = -2, n = 5 and a_n = 0, then the value of a is (CBSE 2011)

The value of a₃₀ – a₂₀ for the A.P 2,7,12,17,….. is (CBSE 2011)

If nth term of an A.P. is (2n + 1), what is the sum of its first three terms? (CBSE 2012)

The sum of first 20 odd natural numbers is: (CBSE 2012)