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.
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Well Done Sir!
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 = 2 6 . 5 7
We know that ∠ C D E = ∠ E A F = 9 0 ∘ and also ∠ D E C + ∠ A E F = ∠ D E C + ∠ E C D = 1 8 0 − 9 0 = 9 0 ∘
⟹ ∠ A E F = ∠ E C D ⟹ △ A E F ∼ △ D E C (AA)
⟹ ∠ E F A = ∠ D E C = 6 3 . 4 3 ∘
Now, prolong C D until it meets f at H
We have ∠ A E F = ∠ H E D , ∠ E A F = ∠ E D H = 9 0 ∘ and, since E is the midpoint of A D , A E = E D ⟹ △ A E F ≅ △ H E D (ASA)
Now it gets interesting: from the congruence between △ A E F and △ H E D and from the fact that ∠ C E F = 9 0 ∘ , we can say that E C is both a median and a height of △ F C H , which means that △ F C H is isosceles, with ∠ C F E = ∠ C H E = ∠ E F A = ∠ D E C = 6 3 . 4 3 ∘ , and that E C is also the bisector of ∠ H C F
⟹ ∠ E C F = 2 ∠ H C F = 2 1 8 0 − 2 × 6 3 . 4 3 = 2 6 . 5 7 ∘
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tan C E D = S / ( S / 2 ) = 2 ⟹ C E D = a r c t a n ( 2 ) = 6 3 . 4 3 .
I n △ E F A , ∠ E F A = 9 0 − ∠ 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 C E h y p o t e n u s e F E = C D E A = S S / 2 = 1 / 2 = tan ( 2 6 . 5 7 ) . B u t i n △ F C E , ∠ E C F = t a n − 1 ( C E F E ) = t a n − 1 ( 1 / 2 ) = 2 6 . 5 7
Sorry I have made changes, it was to make it better.