Another problem from my chinese math practice book

Geometry Level 2

A D = A F AD=AF , A E AE bisects D F DF , A B = 8 AB=8 , the area of triangle A B F ABF is 24, what's the length of E C EC ? And A B C D ABCD is a rectangle.


The answer is 3.

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3 solutions

Noel Lo
Jul 26, 2017

1 2 × A B × B F = 24 \frac{1}{2}\times AB \times BF=24

1 2 × 8 × B F = 24 \frac{1}{2}\times 8\times BF=24

4 B F = 24 4BF=24

B F = 6 BF=6

Considering A B F = 90 \angle ABF=90 degrees, A B = 8 AB=8 and B F = 6 BF=6 tells us that A F = 10 AF=10 employing Pythagoras' Theorem and A D = A F = 10 AD=AF=10 . Moreover, B C = A D = 10 BC=AD=10 by virtue of the fact that A B C D ABCD is a parallelogram. Then C F = B C B F = 10 6 = 4 CF=BC-BF=10-6=4 .

If C E = x CE=x , then D E = C D C E = 8 x DE=CD-CE=8-x . Considering that A D = A F AD=AF and A E AE bisects D F DF , we eventually see that A D E ADE and A E F AEF are congruent. Then E F = D E = 8 x EF=DE=8-x . Now,

C F 2 + C E 2 = E F 2 CF^{2}+CE^{2}=EF^{2}

4 2 + x 2 = ( 8 x ) 2 4^2+x^2=(8-x)^2

16 + x 2 = 64 16 x + x 2 16+x^2=64-16x+x^2

16 x = 48 16x=48

x = 48 16 = 3 x=\frac{48}{16}=\boxed{3}

@Anthony Hong . I have a favour. I really love this Chinese maths book. Can you send this to me at least through pdf to my email hamzjeorge@gmail.com Please I really love that book

Kushagra Sahni
Dec 29, 2015

Did you take the picture from the front camera?

yep...... i did

Anthony Hong - 5 years, 5 months ago

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It became laterally inverted due to that so there was need of a plane mirror. I would prefer if you take the photo from the rear camera instead.

Kushagra Sahni - 5 years, 5 months ago

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Yes sure thanks for your advice :)

Anthony Hong - 5 years, 5 months ago

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