Agreed. Since the radius of each sphere is similar, then the outside of each sphere must touch the center point of the other. In any case, I've been trying to pound out an answer. I found something online that says I should try , but I can't figure out how they came to that conclusion or really even what to do with it.
You can calculate the volume for part of a sphere using discs right? For example, The plot below is a cross-section of two spheres with radius 2 with the properties you required. Suppose you wanted to calculate the volume of the blue part using the disc method. Wouldn't that be:
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Now, can you figure the x-coordinate when the two spheres intersect for arbitrary r, and using the same method, come up with an expression for the total volume common to both spheres with radius r?