You have to sit calmly and quietly and look over this one for a few minutes, until you see what's going on. When the top triangle got flopped over to make the bottom one, the 12-cm side on top made another 12-cm side on the bottom. Also, the short 6-cm piece made another 6-cm piece hooked onto it, and when those are put together, they make 12-cm all together. So now, look at the whole outside triangle that you have, after the two inside ones are put together. The top slanty side is 12-cm, the bottom slanty side is 12-cm, and the straight up-and-down side is also 12-cm. The new big triangle is an equilateral triangle ! What do you know about an equilateral triangle ? -- All three sides are the same length. -- All three angles are the same size. If all three angles are the same size, how big is each one ? Hint: The three angles of every triangle add up to 180 degrees. So angle-y must be 1/3 of 180 = 60 degrees.

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