Equilateral Area

Level pending

There is an equilateral triangle A B C ABC with points D D , E E and F F on lines A B AB , B C BC and C A CA respectively.

A D = 2 B D AD = 2BD , B E = 2 C E BE = 2CE and C F = 2 A F CF = 2AF . If D D , E E and F F are connected to form a triangle and the side lengths of A B C ABC are 62 each, what is the area of D E F DEF to the nearest integer?


The answer is 555.

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1 solution

Lorenc Bushi
Jan 5, 2014

It can be easily proved by congruence of triangles that all the small triangles are congruent to each-other.Therefore the triangle D E F DEF is equilateral,although this is not important to our problem.It's not hard to find that F C = B E = A D = 124 3 FC=BE=AD=\dfrac{124}{3} and B D = E C = A F = 62 3 BD=EC=AF=\dfrac{62}{3} .Lets take a look at the small triangles which are congruent,these are A F D AFD , F E C FEC and B D E BDE .We know that one of their sides is equal to 124 3 \dfrac{124}{3} and the other one is 62 3 \dfrac{62}{3} .We also know the angle between them is 60 60 degrees.Therefore there areas are easy to find using trigonometry. Let S S represent the area of one of these triangles which are congruent.

S = 1 2 124 3 62 3 3 2 S=\dfrac{1}{2}*\dfrac{124}{3}*\dfrac{62}{3}*\dfrac{\sqrt{3}}{2} because s i n 60 = 3 2 sin60=\dfrac{\sqrt{3}}{2} .Multiplying it by 3 3 we get the whole area of the small triangles which is 3 S = 124 62 3 12 3S=\dfrac{124*62*\sqrt{3}}{12} .The area of the larger triangle A B C ABC is equal to 3844 3 4 \dfrac{3844*\sqrt{3}}{4} .If we substract the whole area of the smaller triangles from the areaa of the large we will get our desired result which is approximately 555 \boxed{555}

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