Question

the 6th term of an a.p is zero prove that it's 21st term is triple it's 11th term​

Answer 1

First term = a

Common diff. = d

6th term = a + 5d

a + 5d = 0

a = -5d

21st term = a + 20d = -5d + 20d = 15d

11th term = a + 10d = -5d + 10d = 5d

Therefore,

21st term = 3 × 11th term = 3 × 5d = 15d (proved)