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The difference of the diameter and circumference of base of a cone is $30$ cm. If the slant height of the cone is $25$cm, find the total surface area of the cone.

My attempt,
If $r$ is the radius, $$\frac {r}{2} - 2\pi r=30$$ $$r=\frac {-140}{27}$$.

How does it happen? $(-)$??

1 Answers 1

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Your mistake is d = 2r.

Where d is diameter and r is radius.

So you have |2r - 2πr| = 30.

r = $|\frac{15}{1 - π}|$.

Put the value of r and slant height to solve for total surface area.

  • 0
    r= - 7, how? @Kanwaljit... – 2017-01-21
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    @Newton The circumference is always bigger than diameter, so I think you take the difference the other way – 2017-01-21
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    Yes Ahmed is right circumference is greater than diameter. – 2017-01-21
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    @Ahmed S. Attaalla thank u. – 2017-01-21