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Math Help - Find height of a cone

  1. #1
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    Find height of a cone

    Can some one help me with this.

    A cone has a volume of 81 Pi cubic inches, and the vertx angle of the verticle cross section is 60 degrees. What is the height of the cone?
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  2. #2
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    Quote Originally Posted by SH33 View Post
    A cone has a volume of 81 Pi cubic inches, and the vertx angle of the verticle cross section is 60 degrees. What is the height of the cone?
    First useful fact: If you split the cone into this cross section, you see a triangle. Since it's a cone, the left and right sides must be equal length. What sort of triangle has at least two sides of equal length, and at least one 60 degree angle? (Hint: there aren't a whole lot of other numbers involved in this triangle).
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  3. #3
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    Quote Originally Posted by SH33 View Post
    A cone has a volume of 81 Pi cubic inches, and the vertx angle of the verticle cross section is 60 degrees. What is the height of the cone?
    Second useful fact: the formula for the volume of a cone is V = \frac{1}{3} \pi r^2 h

    Although this doesn't use the length of the side directly, it's very convenient to draw a diagram of the cross section triangle, draw a line representing the height, then figure out formulas for the radius in terms of the side length and height in terms of the side length.

    Once you have your r and h in terms of s, substitute them into the cone formula, solve for s.

    Now that you have s, you can use your s-to-height formula and find the answer.
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