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eroshea

The axis of the cone 2m long makes an angle of 65 degrees with its horizontal base. What is the area of the base if its volume is 5 cu. m?

  • one year ago
  • one year ago

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  1. agent0smith
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    |dw:1355654103593:dw| Guessing it looks like this...

    • one year ago
  2. eroshea
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    but it is not a right triangle so i can't use SOH CAH TOA there

    • one year ago
  3. agent0smith
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    You still can. Notice the height h makes a right angle with the base. |dw:1355654861209:dw| Volume of a cone is 1/3x(area of base)x(height) or \[V = \frac{ 1 }{ 3 } B h\] B is area of the base, h is height. We can find height from the diagram: \[\sin 65 = \frac{ h }{2 }\] so h=2sin65 and we know V = 5 cm^3 \[5 = \frac{ 1 }{ 3 } B \times 2 \sin 65\]

    • one year ago
  4. agent0smith
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    This diagram may help: http://upload.wikimedia.org/wikipedia/commons/d/d2/Cone_3d.png notice the cone on the right, h makes a right angle with the base, and we know the angle that the axis makes with the base.

    • one year ago
  5. eroshea
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    B is 8.28 and B = pi r^2 root 8.28/pi = r ??? is that correct?

    • one year ago
  6. agent0smith
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    Remember that the question is asking for the area of the base, which is the B. It's not asking for the radius.

    • one year ago
  7. eroshea
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    yes.. but i'm just asking whether what i did up there is right or wrong..

    • one year ago
  8. agent0smith
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    I got B as 8.275 m^2, so you're correct. Don't forget the units, metres squared. btw i made a mistake up there, i said V = 5 cm^3 but it's m^3 not cm^3.

    • one year ago
  9. eroshea
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    that's fine.. thanks for helping :))

    • one year ago
  10. agent0smith
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    If you were asked for the radius, then you'd do as you did: \[r = \sqrt{\frac{ 8.275 }{ \pi }}\]

    • one year ago
  11. eroshea
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    thanks :)

    • one year ago
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