There is a right-angle triangle with hypothenus H = 5 cm and side A = 3 cm. Find the length of the third side B using Pythagoras' Theorem.

Pythagoras' theroem states that on a right-angle triangle, the square of the longest side (hypothenus) is equal to the sum of the squares of the other two sides.

Therefore, H² = A² + B² 

5² = 3² + B²

25 = 9 + B²

B² = 25 - 9

B = Square root(16)

B = 4 cm