Question

In the figure, a regular octagon and a regular pentagon are put together. The measure of $\angle ABC$ is

• A. ${171}^{\circ }$
• B. ${117}^{\circ }$
• C. ${127}^{\circ }$
• D. ${107}^{\circ }$

Answer 1

The correct option is B

${117}^{\circ }$

For regular pentagon, n=5.

$\Rightarrow$ Each exterior angle $=\frac{360}{5}={72}^{\circ }$

$={72}^{\circ }$

Now, $e+i={360}^{\circ }$ (Linear pair)

$72+i={180}^{\circ }$

$\Rightarrow i={108}^{\circ }$

For a regular octagon, n=8.

$\therefore$ Each exterior angle $=\frac{360}{8}={45}^{\circ }$

Now, $i+e={180}^{\circ }$ (Linear pair)

$\Rightarrow i+{45}^{\circ }={180}^{\circ }$

$\Rightarrow i={180}^{\circ }-{45}^{\circ }$

$\Rightarrow i={135}^{\circ }$

Now, ${135}^{\circ }+{108}^{\circ }+\angle ABC={360}^{\circ }$ (Angles around a point)

$\Rightarrow \angle ABC+{243}^{\circ }={360}^{\circ }$

$\Rightarrow \angle ABC={360}^{\circ }-{243}^{\circ }$

$={117}^{\circ }$