A rectangular strip 25 cm × 7 cm is rotated about the longer side.

Question:

A rectangular strip 25 cm × 7 cm is rotated about the longer side. Find The total surface area of the solid thus generated.

Solution:

Since the rectangular strip of $25 \mathrm{~cm} \times 7 \mathrm{~cm}$ is rotated about the longer side, we have:

Height, $h=25 \mathrm{~cm}$

Radius, $r=7 \mathrm{~cm}$

$\therefore$ Total surface area $=2 \pi r(r+h)=2 \pi(7)(25+7)$

$=14 \pi(32)$

$=448 \pi \mathrm{cm}^{2}$

$=448 \times \frac{22}{7} \mathrm{~cm}^{2}$

$=1408 \mathrm{~cm}^{2}$

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