In Problems 31-38, find the midpoint of the line segment joining the points P₁ and P2.
31. P₁ = (3, 4); P₂ = (5, 4)
33. P₁ = (−1, 4); P₂ = (8, 0) 35. P₁ = (7, −5); P₂ = (9, 1) 37. P₁ = (a, b); P2 = (0, 0)