Problem 17
Focus on one of the triangles. It has legs of 8 and 15. Use the pythagorean theorem to find the hypotenuse is 17.
Basically you're solving for c in a^2+b^2 = c^2. Use a = 8 and b = 15.
The diameter is 17 and the circumference is
C = pi*d = pi*17 = 17pi
Answer: 17pi
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Problem 18
This figure has four faces, so this is a tetrahedron. The "tetra" is greek that translates to "four".
Another possible name for this figure is triangular pyramid because we have a pyramid with a triangular base.
Answer: Triangular pyramid or tetrahedron