General term:




an = 31/4

n = 10


31/4 = a₁ + (10-1)(d)

31/4 = a₁ + 9d

a₁ = 31/4 - 9d (Equation 1)


an = 71/4

n = 20


71/4 = a₁ + (20-1) (d)

71/4 = a₁ + 19d

a₁ = 71/4 - 19d (Equation 2)


Find d:


31/4 - 9d = 71/4 - 19d

19d - 9d = 71/4 - 31/4

10d = 40/4

10d/10 = 10/10

d = 1 ⇒ the common difference


Fin the first term, a₁:

31/4 = a₁ + (10-1)(1)

31/4 = a₁ + 9

a₁ = 31/4 - 36/4

a₁ = -5/4 ⇒ the first term

paano po nakuha yung 36/4 dito?