Unknown sides, known area

Geometry Level 5

Consider a triangle A B C ABC with points X X and Y Y located on sides A C AC and B C BC which divide the said segments at equal proportions. That is, A X X C = C Y Y B = k \frac{AX}{XC} = \frac{CY}{YB} = k . Locate a point Z Z along X Y XY that shall also divide it in the same manner that points X X and Y Y divide A C AC and B C BC , respectively. That is, X Z Z Y = k \frac{XZ}{ZY} = k as well.

If the area of Δ Z A X = 8 \Delta{ZAX} = 8 and the area of Δ Z Y B = 27 \Delta{ZYB} = 27 , find the area of Δ A B C \Delta{ABC} .


The answer is 125.

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2 solutions

Ahmad Saad
Apr 23, 2017

Efren Medallo
Apr 22, 2017

Let the ratio within the segments be k k .

Now, we can see that A Δ A Y C = k A Δ A B C A_{\Delta{AYC}} = kA_{\Delta{ABC}} , as if we were to treat this side as the base, with the height unchanged, a shift in the base length with some ratio will also result to the same shift in area.

In a similar manner, we can see that A Δ A Y X = k A Δ A Y C A_{\Delta{AYX}} = kA_{\Delta{AYC}} , which in turn implies that A Δ A Y X = k 2 A Δ A B C A_{\Delta{AYX}} = k^2 A_{\Delta{ABC}} . Then, once again, we will find that A Δ A Z X = k A Δ A Y X A_{\Delta{AZX}} = kA_{\Delta{AYX}} , implying finally that A Δ A Z X = k 3 A Δ A B C A_{\Delta{AZX}} = k^3 A_{\Delta{ABC}} . We will use this later.

We can also start at Δ B X C \Delta{BXC} , proceed to Δ B X Y \Delta{BXY} , then to Δ B Z Y \Delta{BZY} . We will eventually arrive at A Δ B Z Y = ( 1 k ) 3 A Δ A B C A_{\Delta{BZY}} = (1-k)^3 A_{\Delta{ABC}} .

Now, observe that

( A Δ Z A X ) 1 3 = k ( A Δ A B C ) 1 3 ( A_{\Delta{ZAX}})^{\frac{1}{3}} = k (A_{\Delta{ABC}})^{\frac{1}{3}}

( A Δ B Z Y ) 1 3 = ( 1 k ) ( A Δ A B C ) 1 3 (A_{\Delta{BZY}})^{\frac{1}{3}} = (1- k)(A_{\Delta{ABC}})^{\frac{1}{3}}

adding them all, we get

A Δ Z A X ) 1 3 + ( A Δ B Z Y ) 1 3 = ( A Δ A B C ) 1 3 A_{\Delta{ZAX}})^{\frac{1}{3}} + (A_{\Delta{BZY}})^{\frac{1}{3}} = (A_{\Delta{ABC}})^{\frac{1}{3}}

and cubing both sides, we arrive at

( A Δ Z A X ) 1 3 + ( A Δ B Z Y ) 1 3 ) 3 = ( A Δ A B C ) (A_{\Delta{ZAX}})^{\frac{1}{3}} + (A_{\Delta{BZY}})^{\frac{1}{3}})^{3} = (A_{\Delta{ABC}})

and so we can now find the area of Δ A B C \Delta ABC , and that is

( 8 1 3 + 2 7 1 3 ) 3 ( 8^{\frac{1}{3}} + 27^{\frac{1}{3}})^3

( 2 + 3 ) 3 (2+3)^3

5 3 = 125 5^3 = \boxed{125}

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