$\"\"$

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(a)

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Radius of the circular sector is $\"\"$ and central angle is $\"\"$ .

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The area of whole circle is $\"\"$ .

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But it is only $\"\"$ part of a circle.

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Therefore the area is $\"\"$ .

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Therefore, the area of the sector is $\"\"$ .

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$\"\"$

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(b)

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The circumference of the base of the cone is $\"\"$ .

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The circumference of the sector of the circle is $\"\"$ which is equal to the circumference of the base of the cone.

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$\"\"$ .

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Therefore, $\"\"$ .

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Substiute the value of $\"\"$ in equation $\"\"$ .

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$\"\"$

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$\"\"$

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(c)

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Continue line $\"\"$ to the axis of revolution, which makes a whole cone.

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Let $\"\"$ be the length of the whole cone.

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Therefore the surface area of the cone is $\"\"$ .

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Now define the ratio :

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$\"\"$

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Substiute the value of $\"\"$ in equation $\"\"$ .

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$\"\"$

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$\"\"$

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(a)

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The area of the sector is $\"\"$ .

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(b)

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$\"\"$ .

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(c)

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$\"\"$ .