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Points D, E, and  are the midpoints of sides BC, CA, and AB, respectively, of triangle ABC. Points X, Y, and Z are the midpoints of EF, FD, and DE, respectively. If the area of triangle XYZ is 21, then what is the area of triangle CXY?

 Aug 3, 2017
 #1
avatar+130466 
+1

 

 

I have down-scaled the problem a little, but the same logic would apply

 

The image below lays out the particulars :

 

The base of triangle XYZ  = XY  = 6  and the height  = 6...so...the area =

6^2 / 2 = 18 sq units

 

Now...X  = (12, 12)  and Y  = (15,6)

 

And  we can call segment XY the base of triangle CXY...and this is

 

sqrt [ (15 -12)^2 + ( 12 - 6)^2 ] = sqrt [ 3^2 + 6^2] = sqrt (45) =  3sqrt (5) units in length = (1)

 

And the equation of the line passing through these two points  is given by

 

y = -2(x -12) + 12 ...  y  = -2x + 36  or      2x + y - 36  = 0   = AX + By + C  = 0

 

Now ....the distance from C to this line is the altitude of CXY and it is given by

 

altitude =  abs (Ax + By + C) / sqrt (A^2 + B^2)   where   (x,y)  are the coordinates of C  = (0,0)

 

So we have that 

 

altitude  = abs ( 2(0) + 1(0)  - 36 ) / sqrt ( 2^2 + 1^2)  =  36 / sqrt (5)  = (2)

 

So.....the area of  triangle CXY  =  base * altitude / 2 =   (1) * (2) / 2 =

[  3 sqrt(5) * 36 ] / [ 2 * sqrt (5) ] =  3 * 36 / 2  =  108 / 2 = 54 sq  units

 

Now.....since in the original problem the area of XYZ is specified as 21 sq units....and the area of XYZ triangle in the diagram = 18 sq units...we can use a proportion to find the true area of CXY....so we have....

 

21 / 18   =  true area of CXY /  54

 

7 / 6  =  true area of CXY / 54

 

true area of CXY = 7 * 54 / 6  =   7 * 9  = 63 sq units

 

 

cool cool cool

 Aug 3, 2017
edited by CPhill  Aug 3, 2017
 #2
avatar+26396 
+1

Points D, E, and F are the midpoints of sides BC, CA, and AB, respectively, of triangle ABC.
Points X, Y, and Z are the midpoints of EF, FD, and DE, respectively.
If the area of triangle XYZ is 21, then what is the area of triangle CXY?

 

Areas = ?

 

ABC=ADEF=14AXYZ=14DEF=1414A=116A=21116A=21A=1621CXY=AB4h2|sin(A)=hAC2+AC4|sin(A)=h34AC|h=34ACsin(A)CXY=AB434ACsin(A)2=332ABACsin(A)|ABACsin(A)=2A=3322A=632A|A=1621=6321621=6221=321CXY=63

 

laugh

 Aug 4, 2017

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