In this movie, this line happened to be wrong.

In Pythagoras' Theorem, he states "The sum of the square roots of any two sides of an isosceles triangle is equal to the square root of the remaining side." In the actual theorem, the squares (not square roots) of the two sides of a right triangle (not isosceles) are summed to give the square of the hypotenuse (the longest side, opposite the right angle). The only time "square root" is used is to run the formula backwards to derive the length of the hypotenuse.

In what movie was this mistake made? Which character?

The scarecrow in "The Wizard of Oz", upon receiving his diploma from the wizard.

Good answer, Professor. Source