The difference of the diameter and circumference

Question

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? $(-)$??

score 2 · Accepted Answer · edited Jan 21 '17 at 08:16

2

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.

edited Jan 21 '17 at 08:16

Ahmed S. Attaalla

18,518

answered Jan 21 '17 at 08:07

Kanwaljit Singh

8,610

r= - 7, how? @Kanwaljit... – pi-π Jan 21 '17 at 08:13
@Newton The circumference is always bigger than diameter, so I think you take the difference the other way – Ahmed S. Attaalla Jan 21 '17 at 08:18
Yes Ahmed is right circumference is greater than diameter. – Kanwaljit Singh Jan 21 '17 at 08:19
@Ahmed S. Attaalla thank u. – Kanwaljit Singh Jan 21 '17 at 08:20

The difference of the diameter and circumference

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