if a + 90 = 180, then a = 90

Start from the conclusion and work back up the proof. Click the arrow to show the parents.

• a = 90,

  if a = b, then b = a

  • 90 = a,

    if 180 + (90 ⋅ (-1)) = y, then 90 = y

    • 180 + (90 ⋅ (-1)) = a,

      if a + 90 = 180, then 180 + (90 ⋅ (-1)) = a

      • a + 90 = 180