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4-25.
  1. A cone has a height of 10 inches and a radius of 4 inches. If a plane cuts the cone h inches above the base of the cone, write an expression for the area of the circular cross-section. Homework Help ✎

Area of a circle = πr2

Sketch the radius and look for a geometric way to find its length.

Do you see the similar triangles?

Similar triangles have proportional sides, write a proportion to solve for r. Be careful! The height of the small triangle is not h.