Example problem of a sphere — lesson. Mathematics CBSE, Class 9.

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The diameter of an orange is

$7 \ cm$ . Find the surface area of

$50$ oranges.

Solution:

Diameter of an orange

$(d)$

$=$

$7 \ cm$

Radius of an orange

$(r)$

$=$

$\frac{d}{2} = \frac{7}{2} = 3.5 cm$

Surface area of a sphere

$=$

$4 \pi r^2$ sq. units

$=$

$4 \times \frac{22}{7} \times {(3.5)}^{2}$

$=$

$4 \times \frac{22}{7} \times 12.25$

$=$

$154$

Surface area of an orange

$=$

$154$

$cm^2$ .

Surface area of

$50$ oranges:

$=$

$154 \times 50$

$=$

$7700$

Therefore, the surface area of

$50$ oranges is

$7700 \ cm^2$ .

Important!

The value of

$\pi$ should be taken as

$\frac{22}{7}$ unless its value is shared in the problem.

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3. Example problem of a sphere