11. GM = avg*sin(arccos(larger-avg))
46. Peaucellier's Linkage- converts rotational motion to linear motion without pistons
	-but only works with alternating rotational source.
	-Actually was invented to draw inversions (Coxeter)
53. I guessed the other way with Steiner Chains (that they could be continuously rotated)
60. 8 planes tangent to 3 spheres of different sizes: top, bottom, under one of each, over one of each
67. cool: a sphere can invert twice, reversing concavity
117. center of mass(intersection of medians) is also known as centroid
136. Can't find cube root with ruler and compass alone
	-fundamental to disproving possibility of other classical construction problems