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IM Geo.3.14 Lesson: Proving the Pythagorean Theorem

What do you notice? What do you wonder?

Elena is playing with the equivalent ratios she wrote in the warm-up.

She rewrites  as . Diego notices and comments, “I got . The  and  remind me of the Pythagorean Theorem.” Elena says, “The Pythagorean Theorem says that . I bet we could figure out how to show that.” How did Elena get from  to ?

What equivalent ratios of side lengths did Diego use to get c?

Prove  in a right triangle with legs length  and  and hypotenuse length .

When Pythagoras proved his theorem he used the 2 images shown here. Can you figure out how he used these diagrams to prove  in a right triangle with hypotenuse length ?

James Garfield, the 20th president, proved the Pythagorean Theorem in a different way.


  • Cut out 2 congruent right triangles
  • Label the long sides b, the short sides a and the hypotenuses c.
  • Align the triangles on a piece of paper, with one long side and one short side in a line. Draw the line connecting the other acute angles.

How does this diagram prove the Pythagorean Theorem?