arithmetic sequence

Question

the sum of 5th and 24th terms of an arithmetic sequence is 50. So, what is the sum of 1st and 28th terms? . what is the sum of first 28 terms?

Answer 1

(a₁+4d) + (a₁+23d) = 50

 

So, 2a₁ + 27d = 50.  

 

      a₁ + (a₁ + 27d) = 50

 

      a_{1 +} a₂₈ = 50

 

S₂₈ = (28/2)(a₁+a₂₈) = (14)(50) = 700