Mathematics Asked on February 17, 2021
The diagram shows an equilateral triangle $ADE$ inside a square $ABCD$ . What is the value of $frac{[Delta ADE]}{[Delta DEC]}$ .
What I Tried: Here is the diagram :-
You can see I marked the angles as required. Now let $AB = x$ . We then have :-
$$[Delta ADE] = frac{sqrt{3}}{4}x^2$$
Now from here :- https://www.quora.com/What-is-the-ratio-of-sides-of-a-30-75-75-angle-triangle-without-sine-rule , I could understand and show that :- $$ED : DC : CE = bigg(frac{sqrt{3} + 1}{2} : frac{sqrt{3} + 1}{2} : 1bigg)$$
So let $EC = k$ , $CD = DE = frac{(sqrt{3} + 1)k}{2}$ .
From here :- $$x = frac{(sqrt{3} + 1)k}{2}$$
$$rightarrow k = EC = frac{2x}{(sqrt{3} + 1)}$$
Now, we can find area by Heron’s Formula. We have :- $$s = x + frac{x}{(sqrt{3} + 1)}$$
$$rightarrow s = frac{xsqrt{3} + 2x}{(sqrt{3} + 1)}$$
So :- $[Delta DEC] = sqrt{s(s-a)(s-b)(s-c)}$
$$rightarrow sqrt{Bigg(frac{(xsqrt{3} + 2x)}{(sqrt{3} + 1)}Bigg)Bigg(frac{x}{(sqrt{3} + 1)}Bigg)Bigg(frac{x}{(sqrt{3} + 1)}Bigg)Bigg(frac{(xsqrt{3})}{(sqrt{3} + 1)}Bigg)}$$
This looks like really a complicated expression, and I really am not going to attempt to simplify this. So can anyone give me a different solution?
Thank You.
If $a,b$ are sides of a triangle and $x$ the mesure of angle between them, then the area of it is $${acdot b cdot sin xover 2}$$
We use that formula here.
We have $AD = AE =DE =DC=a $ so$$frac{[Delta ADE]}{[Delta DEC]} = {{a^2sin 60 over 2}over {a^2 sin 30 over 2}} = sqrt{3}$$
Correct answer by Aqua on February 17, 2021
$DC=x$ and the altitude of $Delta DEC$ from $E$ is $frac{x}{2}$. This gives us $$[Delta DEC] = frac{x^2}{4}$$ And the result $$frac{[Delta ADE]}{[Delta DEC]}=sqrt{3}$$
Answered by Daniel Mathias on February 17, 2021
Answered by Ak. on February 17, 2021
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