It depends on which side is missing. If the hypotenuse (longest side, opposite right angle) is missing, square the lengths of the other two sides, add them, and then square root your answer. If a leg is missing, square the two remaining sides, subtract the smaller from the larger, and square root it. All of this is derived from the Pythagorean Theorem: a^2+b^2=c^2, where a and b are the lengths of the two legs and c is the length of the hypotenuse.
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180 minus two known angle = missing angle. Use Pythagoras' theorem to find its missing side.
You use the Pythagoras Theorem if it is a right-angled triangle. a squared + b squared = h (longest side, diagonal) squared, then square root h to find the longest side. if it is not a right angled triangle, then use the Sine or Cosine rule. Sine rule for: two angles and any one side or two sides and an angle that is not in between the sides. Cosine rule for: all three sides (but then you do not need to find a missing side) or two sides and an angle that is in between.
It made finding the missing side of a right triangle easier
It is the same length as the corresponding side on the other triangle.
One side is not enough. For a right triangle the third side can be calculated by Pythagoras' Theorem if you know the length of any two sides.