In the figure, the outer square's side is \( a + b \). The inner square's vertices touch the outer square's sides, forming right triangles.
The area of the outer square is:
\[ ( a + b )^2 \]
The area of the inner square is:
\[ c^2 \]
The areas of the four shaded right triangles can be calculated as:
\[ 4 \left( \frac{1}{2}ab \right) = 2ab \]
Now, subtract the area of the triangles from the outer square's area:
\[ ( a + b )^2 - 2ab = c^2 \]
Expanding the left side:
\[ a^2 + 2ab + b^2 - 2ab = c^2 \]
This simplifies to:
\[ a^2 + b^2 = c^2 \]
Thus, proving the Pythagorean theorem!