# Calculus

A certain magical substance that is used to make solid magical spheres costs \$800 per cubic foot. The power of a magical sphere depends on its surface area, and a magical sphere can be sold for \$20 per square foot of surface area. If you are manufacturing such a sphere, what size should you make them to maximize your profit per sphere

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1. profit = revenue-cost
revenue = 20*4πr^2 = 80πr^2
cost = 800*4/3 πr^3 = 3200/3 πr^3
so,
profit = 80πr^2 - 3200/3 πr^3
max profit when
160πr - 3200πr^2 = 0
160πr(1-20r) = 0
r = 1/20 ft

looks like marbles make the most money.

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2. Show 1/4 of a set of 8 counters

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