Geometry + Radical Arithmetic = Headache!

Geometry Level 5

Consider a square A B C D ABCD with side of length 4 4 .

Let E E be a point outside A B C D ABCD such that Δ C D E \Delta CDE is equilateral.

Draw C E K = 3 0 \angle CEK = 30^\circ such that ray E K EK intersects ray A C AC at G G , ray D C DC at F F , ray A B AB at P P .

If Area of Δ A G P \Delta AGP can be represented as:

a + b c a + b\sqrt{c} where c c is independent of a perfect square.

Find a + b + c a + b + c .


The answer is 35.

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3 solutions

Chew-Seong Cheong
Mar 10, 2015

I think I was using the method of Nihar Mahajan . It is as follows:

Let the perpendiculars from E E and G G to A P AP be E H EH and G I GI and their lengths be h 1 h_1 and h h respectively, and A P = b AP = b .

We note that A P E = 3 0 \angle APE = 30^\circ , therefore,

P H E H = b 2 h 1 = cot 3 0 = 3 \dfrac {PH}{EH} = \dfrac {b-2}{h_1} = \cot {30^\circ} = \sqrt{3}

b 2 h 1 = b 2 4 + 4 sin 6 0 = b 2 4 + 2 3 = 3 b = 8 + 4 3 \Rightarrow \dfrac {b-2}{h_1} = \dfrac {b-2}{4+4\sin{60^\circ}} = \dfrac {b-2}{4+2\sqrt{3}} = \sqrt{3} \quad \Rightarrow b = 8 + 4\sqrt{3}

We also note that P A G = 4 5 \angle PAG = 45^\circ , therefore, A I = G I = h AI = GI = h , then we have:

P I G I = A P A I G I = b h h = cot 3 0 = 3 b h = 3 h \dfrac {PI}{GI} = \dfrac {AP-AI}{GI} = \dfrac {b-h}{h} = \cot {30^\circ} = \sqrt{3} \quad \Rightarrow b - h = \sqrt{3}h

h = 8 + 4 3 1 + 4 3 = ( 8 + 4 3 ) ( 3 1 ) 2 = 2 + 2 3 \Rightarrow h = \dfrac {8 + 4\sqrt{3}}{1 + 4\sqrt{3}} = \dfrac {(8 + 4\sqrt{3})(\sqrt{3}-1)}{2} = 2 + 2\sqrt{3}

The area of A G P \triangle AGP ,

A = 1 2 b h = ( 8 + 4 3 ) ( 2 + 2 3 ) 2 = 20 + 12 3 A=\frac {1}{2} bh = \dfrac {(8 + 4\sqrt{3})(2 + 2\sqrt{3})}{2} = 20 + 12\sqrt{3}

a + b + c = 20 + 12 + 3 = 35 \Rightarrow a + b + c = 20 + 12 + 3 = \boxed{35}

Almost the same 'method' as yours , only the difference is I dropped just 1 perpendicular and I did not use trigonometry.BTW nice solution.

Nihar Mahajan - 6 years, 3 months ago
Gautam Sharma
Mar 10, 2015

So we know that C E = 4 a n d C G E = 7 5 CE=4 \quad and \quad \angle CGE=75^{\circ}

So applying Sine law in Δ C G E \Delta CGE

C G S i n 30 = 4 s i n 7 5 \displaystyle\frac{CG}{Sin30}=\frac{4}{sin75^{\circ}}

We get C G = 4 2 3 + 1 CG=\frac{4\sqrt2}{\sqrt3+1} .

Hence A G = A C + C G = 4 2 + 4 2 3 + 1 AG=AC+CG=4\sqrt2 +\frac{4\sqrt2}{\sqrt3+1}

= 4 2 ( 3 + 2 ) 3 + 1 =\frac{4\sqrt2(\sqrt3+2)}{\sqrt3+1}

Now A P G = 3 0 applying sine law in Δ A P G \angle APG =30^{\circ}\text{ applying sine law in} \Delta APG

A G S i n 30 = P G s i n 4 5 \displaystyle\frac{AG}{Sin30}=\frac{PG}{sin45^{\circ}}

We get P G = 8 ( 3 + 2 ) 3 + 1 PG=\frac{8(\sqrt3+2)}{\sqrt3+1}

Now a r ( Δ A P G ) = 1 2 × A G × P G × s i n A G P ar(\Delta APG)=\frac{1}{2}\times AG\times PG \times sin\angle AGP

Substituting values we get a r ( Δ A P G ) = 20 + 12 3 ar(\Delta APG)=20+12\sqrt3

Hence 20+12+3=35

I did it by dropping G H A P GH \perp AP . Then using extension of Pythagoras theorem , 45 45 90 45 - 45 - 90 and 30 60 90 30 - 60 - 90 triangles , I went on computing the sides one by one.At last I used 1 2 × b × h \dfrac{1}{2} \times b \times h for area. :)

Nihar Mahajan - 6 years, 3 months ago

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Was it lengthier or smaller? and in problem you should add that c is a prime no. and something like that .i wasted one chance in that as i forgot to simplify it.

Gautam Sharma - 6 years, 3 months ago

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Yeah! Thanks for spotting.

Nihar Mahajan - 6 years, 3 months ago

Absolutely the same way I did it

Rifath Rahman - 6 years, 3 months ago

We know that C E = 4 a n d C G E = 7 5 CE=4 \quad and \quad \angle CGE=75^{\circ} So applying Sine law in Δ C G E \Delta CGE C G 4 = s i n 3 0 o s i n 7 5 . s i n 7 5 o = 6 + 2 4 . C G = 4 2 3 + 1 = 2 2 ( 3 1 ) \dfrac{CG}{4}=\dfrac{sin30^o}{sin75^{\circ}}.~sin75^o=\dfrac{\sqrt6+\sqrt2}{4}.\therefore CG=\dfrac{4\sqrt2}{\sqrt3+1}=2\sqrt2(\sqrt3-1) . A G = A C + C G = 4 2 + 2 2 ( 3 1 ) \therefore AG=AC+CG=4\sqrt2 +2\sqrt2(\sqrt3-1) = 2 2 ( 3 + 1 ) . =2\sqrt2(\sqrt3+1). A G C G = 2 2 ( 3 + 1 ) 2 2 ( 3 1 ) = ( 3 + 2 ) . ( A G C G ) 2 = 7 + 4 3 . . . . . . ( 1 ) \therefore \dfrac{AG}{CG}=\dfrac{2\sqrt2( \sqrt3+1)}{2\sqrt2(\sqrt3-1)} = (\sqrt3+2).~~\therefore~\left(\dfrac{AG}{CG}\right)^2 = 7+4\sqrt3......(1) A r e a Δ G C F = 1 2 C F C G s i n F C G = 1 2 4 2 2 ( 3 1 ) 1 2 = 4 ( 3 1 ) . . . . ( 2 ) A r e a Δ A G P = ( A G C G ) 2 A r e a Δ G C F = ( 1 ) ( 2 ) = ( 7 + 4 3 ) 4 ( 3 1 ) = 20 + 12 3 . 35 Area~\Delta GCF=\dfrac{1}{2}* CF*CG *sin\angle FCG\\=\dfrac{1}{2}*4*2\sqrt2(\sqrt3-1)*\dfrac{1}{\sqrt2}=4*\left(\sqrt3-1\right)....(2)\\Area~\Delta AGP=\left (\dfrac{AG}{CG}\right)^2*Area~\Delta GCF=(1)*(2)\\=( 7+4\sqrt3 )*4*\left(\sqrt3-1\right)= 20+12\sqrt3.\\ \Huge \boxed{\color{#D61F06}{ ~~35~~}}

same solution as yours

Pranav Kirsur - 6 years, 3 months ago

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