Awesome geometry - 9

Geometry Level 3

In square ABCD ,

Point E is the mid point of AD.

Join CE.

Let Point F be on AB such that angle CEF measures 90 .

If measure of angle CED is 63.43 ,

Find the measure of angle ECF. Give your answer to correct 2 decimals.

This problem is a part of set Awesome ' NIHARIAN' geometry


The answer is 26.57.

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3 solutions

There is no need to say angle CED is 63.43. It can be found out as follows. Let S be the side of the square.
tan C E D = S / ( S / 2 ) = 2 C E D = a r c t a n ( 2 ) = 63.43. \tan CED =S/(S/2) = 2 \implies ~ CED = arctan(2)=63.43.\\~~\\~~\\
I n E F A , E F A = 90 F E A = c o m p l i m e n t a r y a n g l e C E D . t w o a n g l e s o f r i g h t a n g l e d s E F A a n d C E D a r e e q u a l . s E F A a n d C E D a r e S I M I L A R . i n s E F A a n d C E D , h y p o t e n u s e F E h y p o t e n u s e C E = E A C D = S / 2 S = 1 / 2 = tan ( 26.57 ) . B u t i n F C E , E C F = t a n 1 ( F E C E ) = t a n 1 ( 1 / 2 ) = 26.57 In~ \triangle EFA ,~\angle EFA = 90 - \angle FEA = complimentary ~angle~\angle CED.\\\implies~two~angles~of ~right~angled~\triangle s~EFA~and ~ CED~ are~ equal.\\\therefore~\triangle s~EFA ~ and ~ CED~are~\color{#D61F06}{SIMILAR}.\\\therefore~~in~\triangle s~~EFA~and~CED,~\dfrac{ hypotenuse ~ FE}{ hypotenuse ~CE}\\= \dfrac{EA}{CD}= \dfrac{S/2}{S}=1/2=\tan(26.57).\\But~in~ \triangle~FCE,~ \angle ECF~=~tan^{-1}~( \dfrac{FE}{CE} )=tan^{-1}(1/2)\\=\boxed{ \color{#20A900}{ 26.57}}
Sorry I have made changes, it was to make it better.


Well Done Sir!

Nihar Mahajan - 6 years, 4 months ago
Nguyen Thanh Long
Jan 31, 2015

It is easy to realize that two triangles CEF and CED are uniform. So: C E D = E F C E C F = 9 0 0 E F C = 26.57 \angle{CED}=\angle{EFC} \Rightarrow \angle{ECF}=90^{0}-\angle{EFC}=\boxed{26.57}

Rick B
Feb 2, 2015

We know that C D E = E A F = 9 0 \angle CDE = \angle EAF = 90^\circ and also D E C + A E F = D E C + E C D = 180 90 = 9 0 \angle DEC + \angle AEF = \angle DEC + \angle ECD = 180-90 = 90^\circ

A E F = E C D A E F D E C \implies \angle AEF = \angle ECD \implies \triangle AEF \sim \triangle DEC (AA)

E F A = D E C = 63.4 3 \implies \angle EFA = \angle DEC = 63.43^\circ

Now, prolong C D \overline{CD} until it meets f f at H H

We have A E F = H E D \angle AEF = \angle HED , E A F = E D H = 9 0 \angle EAF = \angle EDH = 90^\circ and, since E E is the midpoint of A D \overline{AD} , A E = E D A E F H E D \overline{AE} = \overline{ED} \implies \triangle AEF \cong \triangle HED (ASA)

Now it gets interesting: from the congruence between A E F \triangle AEF and H E D \triangle HED and from the fact that C E F = 9 0 \angle CEF = 90^\circ , we can say that E C \overline{EC} is both a median and a height of F C H \triangle FCH , which means that F C H \triangle FCH is isosceles, with C F E = C H E = E F A = D E C = 63.4 3 \angle CFE = \angle CHE = \angle EFA = \angle DEC = 63.43^\circ , and that E C \overline{EC} is also the bisector of H C F \angle HCF

E C F = H C F 2 = 180 2 × 63.43 2 = 26.5 7 \implies \angle ECF = \dfrac{\angle HCF}{2} = \dfrac{180-2 \times 63.43}{2} = \boxed{26.57^\circ}

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