Question

21. Write the expression an-ay for the A.P. a, a + d, a + 2d,
Hence, find the common difference of the A.P. for which
(i) 11th term is 5 and 13th term is 79.
(ii) a10 - a5 = 200
(iii) 20th term is 10 more than the 18th term.​

Answer 1

Formula used in all parts

An=a+(n-1)d

1)a11=5

A13 =79

a+10d=5

a+12d=79

On solving we get d=37

2) a10-a5=200

a+9d-a-4d=200

d=40

3)a20=10>A18

a+19d>a+17d

d>0

a<10

We can't find value of d only get relation in d that is d>0