In a right triangle, a² + b² = c². c is the hypotenuse, the side across from the right angle. Legs 3 and 4 give hypotenuse 5. The short list is in ACT Math formulas.
Right angle first
If there is no right angle, do not use the theorem. Special 30-60-90 and 45-45-90 ratios are faster when the figure matches, in 30-60-90 triangles. Coordinate distance is the same idea on a grid, in the distance formula.
The midpoint of a segment is a different job, in the midpoint formula.
The bottom line
Right triangle: legs squared plus legs squared equals hypotenuse squared.
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