Using Pythagoras Theorem, the third side of a right-angled triangle can be calculated when two sides are given.

Suppose A = length of hypotenuse and
B & C = lengths of the sides containing the right angle

Then (A^2) = (B^2)+(C^2)

Proof:

If a = angle opposite side A ( =90 degrees)

b = angle opposite side B

c = angle opposite side C

then B = A sin a and C = A cos a

Squaring and adding,we get the result.

Simply put, in a triangle, the square of the hypoteneuse is equal to the sum of the squares of the other two sides. Simple as that.

A fucking useless maths thing that u will never need to use.