Trapped Triangle

Geometry Level 2

In the above picture, X and Y are midpoints of DC & BC. Area of triangle AXY can be expressed as S times of the area of ABCD. Find S*8. ABCD is a //gm


The answer is 3.

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7 solutions

Horisadi Afyama
Feb 7, 2015

Nick Byrne
Feb 7, 2015

Assuming ABCD to be a parallelogram:

-Let U be the midpoint of AB , V the midpoint of AD and O the midpoint of both UX and VY .

-As Y is the midpoint of BC , ABY is 1 2 \frac{1}{2} of ABYV , which is 1 2 \frac{1}{2} of ABCD ., thus ABY is 1 4 \frac{1}{4} of ABCD .

-Similarly as X is the midpoint of AD , ADX is 1 2 \frac{1}{2} of ADXU which is 1 2 \frac{1}{2} of ABCD ., thus ADX is 1 4 \frac{1}{4} of ABCD ..

- CXOY is 1 4 \frac{1}{4} of ABCD . and CXY is 1 2 \frac{1}{2} of this area so CXY is 1 8 \frac{1}{8} of ABCD .

Finally AXY = (1- ( 1 4 \frac{1}{4} + 1 4 \frac{1}{4} + 1 8 \frac{1}{8} )) x ABCD , so S = 3 8 \frac{3}{8}

Thus 8 × \times S =3

AXY =[ 1- (1/4 +1/4 +1/8) ] * ABCD

Sahil Jindal - 6 years, 4 months ago
Mohan Kisku
Feb 9, 2015

Let a be the height of ABCD; b be the length CD=AB; ar of ABCD = ab; ar of triangle ADX=ab/4; ar of triangle XCY= ab/8; ar of triangle ABY= ab/4; ar of triangle AXY= Sab=ab-(ab/4)-(ab/8)-(ab/4)=3ab/8; S=3/8;S 8=(3/8) 8=3

Harish Pasapula
Feb 8, 2015

If area of //gm is Q, Area of ADX is Q/4. Area of ABY = Q/4. Area of XCY = Q/4. Remaining area = Q-Q/4-Q/4-Q/8=3*Q/8

Shahzad Karim
Feb 8, 2015

Area of the given triangle is equal to the area of the parallelogram minus sum of areas of the remaining three triangles. And, two of the triangles share a common base (with the parallelogram) but half the altitude, so their areas are both 1/4 of parallelogram, and the third triangle has both the base and the altitude half of those of parallelogram so its area is 1/8 of that of parallelogram. Therefore, required area = (1-1/4-1/4-1/8)xparallelogram.

Assume the longest side is 4 units and smaller side is 2 units. So area of parallelogram is 8 Sq.units.

X is mid point of larger side, so BX =2, XC=2

Y is mid point of smaller side, so CY=1and ZB=1.

Area of 🔺 ADX =1/2(3

Explained simply:

Axyz = ab bc - ab bc/4 - ab bc/4 - ab bc/8

Axyz = ab bc - (2 ab bc/8 + 2 ab bc/8 - ab bc/8)

Axyz = 3ab*bc/8

Axyz/Aabcd = 3/8

Times 8.... --> 32x 2)=2 Sq.units Similarly Area of 🔺 XCY = 1. Sq unit And area of 🔺 ABZ =2 Sq units

Area of square - area of these 3 triangles give area of 🔺 AXZ . i .e = 8-2-2-1=3 Sq units .

As per given relation, AREA AXY= S (AREA ABCD) 3= S X 8 S= 3/8

So 8s=3

OR

Axyz = ab x bc - ab x bc/4 - ab x bc/4 - ab x bc/8

Axyz = ab x bc - (2 x ab x bc/8 + 2 x ab x bc/8 - ab x bc/8)

Axyz = 3ab x bc/8

Axyz/Aabcd = 3/8

Times 8.... --> 3

Caleb Townsend
Feb 5, 2015

Assuming A B C D ABCD is a parallelogram, the value of S S can be proven as follows.
Join B B and D D to form segment B D BD which is parallel to X Y XY . The triangles Δ X Y C \Delta XYC and Δ D B C \Delta DBC are therefore similar, and since the side lengths of Δ X Y C \Delta XYC are half those of Δ D B C \Delta DBC , its area must be 1 4 \frac{1}{4} of Δ D B C \Delta DBC , or 1 8 \frac{1}{8} of A B C D ABCD .

Join A A and C C now to form segment A C AC . Since B Y BY is half the length of B C BC , the area of Δ A B Y \Delta ABY is half that of Δ A B C \Delta ABC , or 1 4 \frac{1}{4} of A B C D ABCD . Similarly, the area of Δ A B X \Delta ABX is 1 4 \frac{1}{4} of A B C D ABCD . Therefore, the answer is

S = 1 1 4 1 4 1 8 = 3 8 S = 1 - \frac{1}{4} - \frac{1}{4} - \frac{1}{8} = \frac{3}{8} 8 S = 3 8S = \boxed{3}

It's also easy if you assume S S is a constant for all parallelograms; you can then just assume A B C D ABCD is a square of side length 2 2 , and find the area of Δ A X Y \Delta AXY is 4 1 1 1 2 = 3 2 . 4 - 1 - 1 - \frac{1}{2} = \frac{3}{2}. Therefore S = ( 3 2 ) 4 = 3 8 S = \frac{(\frac{3}{2})}{4} = \boxed{\frac{3}{8}}

Caleb Townsend - 6 years, 4 months ago

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