Area of a triangle $=\frac{1}{2}\times BASE \times HEIGHT $
In the given figure hexagon $ABCDEF$ is equiangular, $ABJI$ and $FEHG$ are squares with areas $18$ and $32$ respectively, $\triangle JBK$ is equilateral and $FE=BC$. What is the area of $\triangle KBC$?
$\textbf{(A) }6\sqrt{2}\quad\textbf{(B) }9\quad\textbf{(C) }12\quad\textbf{(D) }9\sqrt{2}\quad\textbf{(E) }32$.
AMC 8 2015 Problem 21
Area of Squares and triangles.
6 out of 10
Challenges and Thrills of Pre college Mathematics
First hint
Can you find the lengths of one side of the squares $ABJI$ and $FEHG$ ??
If you can, then try to use the given conditions to find out the lengths of two sides of $\triangle KBC$
Second Hint
Area of a square =$(\textbf{Side of the square})^2$
Then Side of a square =$\sqrt{(\textbf{Area of the square})}$
Then you can easily find the lengths of a side of the squares $ABJI$ and $FEHG$ which is $3\sqrt{2}$ and $4\sqrt{2}$ respectively.
Third Hint
Then by the given condition $\overline{FE}=\overline{BC}=4\sqrt{2}$
Since $\triangle JBK$ is an equilateral triangle then all of its sides are equal.
and $\overline{JB}=\overline{BK}=3\sqrt{2}$
Now as you know that $\triangle JBK$ is equilateral and the hexagon $ABCDEF$ is equiangular. Can you find out the measure of $\angle KBC$ ??
Final Step
From the figure we can clearly see $\angle JBA + \angle ABC + \angle KBC + \angle KBJ = 360^{\circ}$
$\angle KBC = 360^{\circ}-90^{\circ}-120^{\circ}-60^{\circ}$ [Since $\angle JBA=90^{\circ}$ (an angle of a square) $\angle ABC=120^{\circ}$ (an angle of an equiangular hexagon) and $\angle JBK= 60^{\circ} $(an angle of an equilateral triangle)]
i.e., $\angle KBC= 90^{\circ}$
Then $\triangle KBC$ is a right angle triangle
Then the base and height of $\triangle KBC$ are $BC$ and $KB$
So the area of $\triangle KBC=\frac12 \times KB \times BC = \frac12 \times 4\sqrt{2} \times 3\sqrt{2} = 12$