Length Ratio

Geometry Level 4

A B C D ABCD is a parallelogram. E E is a point on A B AB such that 234 × A E = E B 234 \times AE = EB . Let D E DE intersect A C AC at F F . What is the ratio A C : A F AC:AF ?


The answer is 236.

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11 solutions

Mayank Bhutani
May 20, 2014

A B C D ABCD be a parallelogram and A C AC as diagonal firstly through A A , we will draw a line parallel to D E DE . Let it be A Z AZ and it will meet the extended C D CD at Z Z . Now A E D Z AEDZ is a parallelogram. A E AE is equal to Z D ZD . let A E AE be 'x'. Z D ZD is also x, E B EB is 234x. so, A B AB is 235 x 235x . A B AB is equal to C D CD so C D CD is 235 x 235x . also Z C ZC is Z D + D C ZD+DC which is x + 235 x = 236 x x +235x =236x . In triangle A C Z ACZ by thales theoram, A C A F \frac {AC}{AF} is equal to Z C Z D \frac {ZC}{ZD} so f r a c A C A F frac {AC}{AF} is equal to 236 x x = 236 \frac {236x}{x} = 236 .

Kevin Sun
May 20, 2014

Draw BD and let it intersect AC at Q. We thus know that AQ = QC and BQ =QD. Also, by menelaus, we have that AE/EB * BD/DQ * QF/FA = 1. Thus 1/234 2 QF/FA =1, or QF/FA = 118. Thus AC/AF = 2 (QF+FA)/FA = 2+2 117 = 236.

Carlo Nuñez
May 20, 2014

Let AE = x and EB = 234x. Then, DC = AB = AE + EB = 235x. Notice that triangle AEF and triangle CDF are similar, so,

AE / AF = DC / CF x / AF = 235 x / CF CF / AF = 235.

But we need to find AC / AF, or equivelantly as

(AF + CF)/AF = 1 + CF/AF = 1 + 235 = 236

Ng Zhen Hao
May 20, 2014

Let the length of BE be 234x, then AE = x. Hence AB = DC = 235x. Triangle AEF is similar to triangle DCF, since angle AEF = angle DCF, angle AFE = angle DFC, and angle FAE = angle FDC. Since AE = x and DC = 235x, then AE : DC = AF : FC = 1 : 235, thus AC = 235 + 1 = 236 units. Thus AC : AF = 236

suppose AE = x, EB = 234x. (AB=235x) AF : FC = 1 : 235 ; FC : AF = 235 AC = AF + FC AC : AF = (AF + FC) : AF = 1 + 235 = 236 So, AC : AF is 236

Shourya Pandey
May 20, 2014

we have EB=234 AE, so AB=235 AE,i.e., DC=235 AE. triangles AFE and FCD are similar, because A F E \angle AFE = F C D \angle FCD and F A E \angle FAE equals F C D \angle FCD , being alternate. So AF/FC = AE/DC=1/235, so FC=235 AF, so AC=236*AF. so the required ratio is 236.

Marcos Kawakami
May 20, 2014

Let A E = x AE = x . Then E B = 234 x EB = 234x .

Since A B D C AB \parallel DC , E A C = A C D \angle EAC = \angle ACD and A E D = E D C \angle AED = \angle EDC \Rightarrow triangles F A E FAE and F C D FCD are similar. Thus, D C A E = F C A F \frac{DC}{AE} = \frac{FC}{AF} . But D C = A B = 235 x DC = AB = 235x (because A B C D ABCD is a parallelogram), so D C A E = 235 1 = F C A F 235 × A F = F C \frac{DC}{AE} = \frac{235}{1} = \frac{FC}{AF} \Rightarrow 235\times AF = FC \Rightarrow A C : A F = ( A F + F C ) : A F = 236 A F : A F = 236 AC : AF = (AF + FC) : AF = 236AF : AF = \boxed{236}

Michael Ma
May 20, 2014

Construct Line EG parallel to AD such that G is on AC. Then AG:CG=1:234 and then EG:AD=1:235. So AF:FG=235:1. Then AF:FG:GC=235:1:234*235. Then AC:AF=236.

AF=x, FC=234x+x, AC=236x

Sayan Chowdhury
May 20, 2014

just clearly draw the perfect picture at first... you have drawn the following straight lines :DE,AC. now we know from the parallelogram ABCD that AB||DC and the straight line DE is sector so angle(AED)=angle(EDC) [alternate angle]and likewise when AC is the sector then angle(BAC)=angle(ACD).[alternate angle]....... now taking the two triangle AEF and triangle DFC::we get,,, 1)angle(AEF)=angle(FDC).2)angle(EAF)=angle(FCD). 3)angle (AFE)=angle(DFC)[vertically opposite].....so triangle AEF and triangle DFC are equal...hence [AE/DC]=[AF/FC] =>[DC/AE]=[FC/AF] =>[DC/AE]+1=[(FC+AF)/AF]=[AC/AF] =>[AC/AF]=[234 (DC/EB)]+1 Now AE/EB=1/234 So (AE+EB)/EB=1+(1/234) =>AB/EB=235/234 =>DC/EB=235/234 SO COME TO PREVIOUS RESULT:::[AC/AF]=[234 (DC/EB)]+1 =[234*(235/234)]+1 =235+1=236
here comes the answer,,,, thanking you ,,,,,,,,,,,,,/-

Calvin Lin Staff
May 13, 2014

Since A B C D AB \parallel CD , we have: E A F = F C D \angle EAF = \angle FCD and A E F = F D C \angle AEF = \angle FDC . By opposite angles, E F A = C F D \angle EFA = \angle CFD . Thus, by angle-angle-angle, triangles F A E FAE and F C D FCD are similar. Therefore, F C F A = C D A E = A B A E \frac{FC}{FA} = \frac{CD}{AE} = \frac{AB}{AE} , where the final equality follows since A B C D ABCD is a parallelogram and A B = C D AB = CD .

So we have, A C = A F + F C A C A F = 1 + F C A F AC = AF + FC \Rightarrow \frac{AC}{AF} = 1 + \frac{FC}{AF}\ . Substituting the above relation for F C A F \frac{FC}{AF} , we have: A C A F = 1 + A B A E \frac{AC}{AF} = 1 + \frac{AB}{AE} = 1 + A E + E B A E = 1 + \frac{AE + EB}{AE} = 1 + 1 + E B A E = 1 + 1 + \frac{EB}{AE} = 2 + 234 = 236 = 2 + 234 = 236 .

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