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A solid cone and a solid cylinder have equal base and equal height. If the radius and height be in the ratio of 12 : 5, the ratio of the total surface area of cone to that of the cylinder is

Solution

✅ Correct Option: 4

Let r=12kr=12k and h=5kh=5k.

The cone needs its slant height, found by Pythagoras:

l=r2+h2=144k2+25k2=169k2=13kl=\sqrt{r^2+h^2}=\sqrt{144k^2+25k^2}=\sqrt{169k^2}=13k


SolidTotal surface area formulaValue
Coneπr(r+l)\pi r(r+l)π(12k)(12k+13k)=300πk2\pi(12k)(12k+13k)=300\pi k^2
Cylinder2πr(r+h)2\pi r(r+h)2π(12k)(12k+5k)=408πk22\pi(12k)(12k+5k)=408\pi k^2

Ratio=300πk2408πk2=300408=2534\text{Ratio}=\dfrac{300\pi k^2}{408\pi k^2}=\dfrac{300}{408}=\dfrac{25}{34}

So the ratio is 25:3425:34.

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