Where do you see lots of Asians?

Geometry Level 5

A B C ABC satisfies B = 9 0 , A = 6 5 \angle B=90^{\circ}, \angle A=65^{\circ} . Let D , E , F D,E,F be on segments B C , A C , A B BC,AC,AB respectively such that E F = B D = A B EF=BD=AB and E F B C EF||BC . Let A D E F = G AD\cap EF=G and H H is the foot of projection from G G onto B C BC , Find B A H \angle BAH in degrees.


Answer to title: UCLA, because the university acronym could stand for U(you) C(see) L(lots) of A(asians), and the place is indeed known for having a large percentage of asian students.


The answer is 25.

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3 solutions

Ahmad Saad
Oct 25, 2015

Stewart Gordon
Oct 11, 2015

Since B D = F E B C BD = FE || BC , D D lies on B C BC , and F F lies on A B AB , you can deduce that B D E F BDEF is a rectangle snugly against A B AB and B C BC .

F G E FGE and B D C BDC are both straight lines, and B F BF , D G DG and C E CE all meet at A A . From this, we deduce that G G splits F E FE in the same ratio as D D splits B C BC . In other words, F E F G = B C B D . \frac{FE}{FG} = \frac{BC}{BD}. But F E = B D FE = BD by construction, therefore B C . F G = ( F E ) 2 . BC.FG = (FE)^2.

If we let A B = 1 AB = 1 , then F E = 1 FE = 1 by construction, and B C = tan 6 5 BC = \tan 65^\circ . Then we have F G = cot 6 5 = tan 2 5 . FG = \cot 65^\circ = \tan 25^\circ. But B H G F BHGF is a rectangle, therefore F G = B H FG = BH . So B H = tan 2 5 B H B A = tan 2 5 B A H = 2 5 . BH = \tan 25^\circ \\ \Rightarrow \frac{BH}{BA} = \tan 25^\circ \\ \Rightarrow \angle BAH = \boxed{25^\circ}.

Kenny Lau
Oct 18, 2015

(Coordinate Geometry FTW)

(Please draw a diagram to comprehend this)

Let B be origin, A be (0,1), C be ( tan 6 5 \tan65^\circ ,0).

Then, eqn. of AC: ( tan 6 5 ) y + x = tan 6 5 (\tan65^\circ)y+x=\tan65^\circ .

D is (1,0), BDEF is a rectangle, so x-coordinate of E is 1.

E lies on AC, so sub. x=1 to eqn. of AC to get y = tan 6 5 1 tan 6 5 y=\dfrac{\tan65^\circ-1}{\tan65^\circ} .

That is the y-coordinate of E.

Eqn. of AD: x + y = 1 x+y=1 .

Eqn. of EF: y = tan 6 5 1 tan 6 5 y=\dfrac{\tan65^\circ-1}{\tan65^\circ} .

Solve, x = 1 tan 6 5 = tan 2 5 x=\dfrac1{\tan65^\circ}=\tan25^\circ .

Therefore, H is ( tan 2 5 \tan25^\circ ,0).

BAH is a right-angled triangle, and tan B A H = tan 2 5 1 \tan\angle BAH=\dfrac{\tan25^\circ}1 .

Therefore, B A H = 2 5 \angle BAH=25^\circ .

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