Question

If 5, a₂, a₃,...., a₂₀, 145 is an A.P., then a₂ + a₂₀ = ______.

Answer 1

If 5, a₂, a₃,...., a₂₀, 145 is an A.P., then a₂ + a₂₀ = 150.

Explanation:

5 is a₁ and 145 is a₂₁ (there are 21 terms).

Using the nth-term formula a_n = a₁ + (n – 1)d, we have

145 = 5 + 20d

So d = 7. 

Then a₂ = 5 + 7 = 12 and a₂₀ = 5 + 19 × 7 = 138.

So a₂ + a₂₀ = 12 + 138 = 150.