(7) If 9th term of an A.P is zero, prove that its 29th term is double (twice) the 19th term

Solution:

9th term = 0

           t₉ = 0

  a + 8 d = 0

          a = -8d

t₂₉ = a + 28 d

    = -8 d + 28 d

   = 20 d

  = 2[10 d]

   = 2 [-8d + 18 d]

  = 2[a + 18 d]

t₂₉ = 2[t₁₉]