Question

the 6th term of an AP is zero. prove that its 21st term is triple its 11th term

Answer 1

Hey friend, ☺ here is ur answer __________________________

a6 = 0
a + 5d = 0 .....…....(1)

a21 = a + 20d
= a + 5d + 15d
= 0 + 15d ( from (1) )
a21 = 15d

a11 = a + 10d
= a + 5d + 5d
= 0 + 5d ( from (1) )
a11 = 5d...............(2)

we can also write a21 as :-
a21 = 15d
= 3 × 5d
a21 = 3 + a11. from (2)

hence proved