What is the Pythagorean Theorem?
The Pythagorean Theorem is a mathematical formula that states that in a right triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. The theorem is expressed mathematically as:

c^2 = a^2 + b^2,

where c is the length of the hypotenuse, and a and b are the lengths of the other two sides.

The Pythagorean Theorem is named after the ancient Greek mathematician Pythagoras, who is credited with its discovery, although it is likely to have been known by the Indians as well. The theorem has many practical applications in fields such as engineering, architecture, and navigation, and it is a fundamental theorem in Euclidean geometry.
Could you give an example on how to use this formula?
Sure! 

imagine a right sided triangle with the longest side of 5 cm, and a second side with 4 cm. According to the Pythagorean theorem the third side would have a length of 

a² + b² = c²
c² - b² = a²
5² - 4² = a²

a = 3cm.

With this formula you are able to calculate the third side of a right sided triangle.
How can you use this in complex AC power calculations ?