Are you allowed to use trigonometry? Or is this a purely geometric problem?
In the figure below, A, G, B, F, and C are collinear; DB is perpendicular to BE; DG is perpendicular to AC; and EF is perpendicular to AC. If DB is 20 units long, EB is 10 units long, and EF is 8 units long, how many units long is DG?
I know that the answer is 12 but I'm having trouble justifying why the triangle are similiar, I just don't see the proof of it
Ahh that's even more confusing ackbeet Not sure how to do that since we don't know any other angle measures--those angles could be anything!
Archie--well sure it's obvious that the rest of the angles sum to 90 LOL But how do I prove the triangles similiar? AA, SAS.. that type of thing. What's the proof that is..
Read Archie's post again...he's spot on!
Let k = angle EBF, then angle BEF = 90-k ; OK?
Since angle DBE = 90 (DB perpendicular to EB), then angle DBG = 180-90-k = 90-k ; OK?
Leaves angle BDG = k ; so triangles similar; OK?
Edit...thanks Soroban; but I beat you by .0001333... second !
Wow, that's something I never learned before to prove similiar triangles.. I thought it has to be one of those "congruences" type proofs, like HL... I learned something today For hours I was playing around with DG and EF being parallel looking for corresponding angles or something
EDIT: How do you conclude angle GDB = theta Soroban? What about the 90 - theta?
Wilmer--same thing sort of--why is k involved in the equation of DBG saying that it's 90 - k. Why does it have to be k? We don't know anything about that triangle having a "k" angle. Hmmm I thought I got it too
A triangle's 3 angles total 180 degrees, right?
IF a right triangle, then 1 angle = 90 degrees; so other 2 angles total 90 degrees, right?
So if we let 1 of those 2 angles = k (or whatever variable that turns you on!),
then the other angle HAS TO equal 90 - k, right?
if you imagine rotating triangle EBF anticlockwise about the point B
until E is on the other triangle's DB side,
then EF will be parallel to GB,
BF will be parallel to GD
and we can see that angle BEF equals angle DBG
and that angle FBE equals angle BDG.
The triangles are right-angled, so both triangles have the exact same angles,
hence they are similar.