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In acute-angled triangle ABC, let D be the foot of the altitude from A, and E be the midpoint of BC. Let F be the midpoint of AC. Suppose 6 BAE = 40◦ . If 6 DAE = 6 DF E, what is the magnitude of 6 ADF in degrees?

kes1968  Jun 6, 2017
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4+0 Answers

 #1
avatar+520 
+1

Are these spurious 6s supposed to be angle symbols, viz., should it read:

 

If ∠DAE = ∠DFE, what is ....

 

 

 

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Badinage  Jun 6, 2017
edited by Badinage  Jun 6, 2017
edited by Badinage  Jun 6, 2017
 #2
avatar+6900 
+2

Because D and F are the mid-points of BC and AC respectively, DF || AB. Therefore, angle AEF should equal 40 deg.

Let G be the intersection point of AD and EF.

Angle DAE + 40 deg = angle AGF

Angle DFE + angle ADF = angle AGF

Because angle DAE = angle DFE,

angle ADF should equal 40 deg :D

MaxWong  Jun 6, 2017
 #3
avatar+79894 
+2

 

Max.....E is the mid-point of AB, not D....so.....  CE/EB = CF/FA ....

 

Which would mean that EF ll AB.....instead of  DF ll AB .....

 

Then angle FED  = angle ABE   and angle GDE = angle ADB

 

So by  AA congruency

 

Triangle  DGE  is similar to triangle DAB

 

Thus....angle DGE  =  angle DAB

 

And angle DAB =   angle DAE + angle EAB

 

So.....this means that

 

Angle DGE  =  angle DAE + angle EAB     

 

But, by the exterior angle theorem,  angle DGE  also  = angle DFE + angle GDF

 

And angle DAE  = angle DFE

 

So.....this means that

 

angle DAE  + angle EAB   =  angle DFE + angle GDF

 

Subtracting like angles on either side of the equation, we have that

 

angle EAB  =  angle GDF

 

But angle EAB  = 40°....so  angle GDF  has the same measure

 

But  angle GDF  =  angle ADF

 

And Max's assertion that angle ADF = 40°  is correct, after all  !!!

 

**Thanks, Max, for some of the hints you gave.....you actually did 90% of the proof....I just "cheated" off you a little.....LOL!!!!!!

 

Here is a "quasi" pic :

 

https://gyazo.com/7bea17ad8e37aa35ee2230ca584f6f47

 

 

 

 

 

 

 

 

cool cool cool

CPhill  Jun 6, 2017
edited by CPhill  Jun 6, 2017
edited by CPhill  Jun 6, 2017
 #4
avatar+5576 
+1

Here is CPhill's picture: smiley

 

hectictar  Jun 6, 2017
edited by hectictar  Jun 6, 2017

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