Areally simple question that i just don't understand.

The key is that Pythagoras' Theorem works in both directions.

-- IF it's a right triangle, THEN [ A² + B² = C² ].
and
-- IF [ A² + B² = C² ], THEN it's a right triangle.

Look at the triangle on the sheet.

Left side:
length of the side is 0.75
square of the left side = (0.75)² = 0.5625

Bottom side:
length of the side = 1
square of the bottom side = (1)² = 1

Sum of the squares = (0.5625 + 1) = 1.5625

Square root of that sum = √1.5625 = 1.25

Is that the length of the third side ?  Yes !
Then [ A² + B² = C² ].
Then it's a right triangle.
You proved it.
yay

22.