the fastest approach is by 30:60:90 angle formed by BAE
do it quick by 1:root3:2
you get root3/2 in quantity A as well as in quantity B
so c is the answer

is it 30-60-90 or 45-90-45?

Can you show in details

Thank you.

Can Someone Pls explain how we know that BA = BD...or just how BDAE is a square.

and AB=DE = x

So quadrilateral ABDE is a square.

To be a square AB = AE has to be given. Not AB=DE.

Plz see the attached diagram,


In the quadrilateral ABDE,

we have AB = DE and angle A = angle E = 90 degree and the diagonal bisect each other at equal length . SO angle b = angle d = 90

Now the triangle ABF and triangle BDF are equal, since they both have the common same height

i.e AB = BD.

Dear friend,
how do you get 30:60:90, please explain, really need your help.
thanks

I seem to be the only one who thinks B is greater. My reasoning is thus:

They both have same lengths for Base and Height. However, Area of a Triangle is 1/2bh which makes it smaller than bh (Area of a quadrilateral).

@poster can you provide the solution for this question?

Please, see the explanation above.

Regards

I still do not understand how is ABDE a square. How do we know AB=AE or AB = BD? All angles are 90 degrees, which means it can be a rectangle.

We have no way of knowing whether quadrilateral BCDF is a square or not, we also have no way of knowing whether ABE is a 30-60-90 triangle. Fortunately, these details don't matter. Observe:

The area of triangle ABE is \(\frac{AB*AE}{2}\). Now, note that AB=DE and since CD || BE and BC || DF, we must have BC = BF = DF = CD. We can then infer that CF projects orthogonally down onto AE, meaning its length is the same as DE which forms a right angle with AE. Thus, CF = DE = AB. We then can subdivide the quadrilateral BCDF into four equal sized right triangles each with base \(\frac{AE}{2}\) and height \(\frac{AB}{2}\), giving each an area of \(\frac{AE*AB}{8}\). We then multiply this quantity by four to get \(\frac{AB*AE}{2}\), the same as the area of triangle ABE.

Hi,

Could you plz let me know how you have proved BC = BF = DF = CD as only AB=DE and since CD || BE and BC || DF is mentioned?


Moreover can we really infer " that CF projects orthogonally down onto AE "? unless BCDF is a rhombus,square,parallelogram

Certainly. Since AB=DE, CD || BE, and BC || DF, we have BC=CF=DF=CD due to the statement "If two parallel lines intersect two parallel lines, each of the resulting segments will be equal to their opposite." This is equivalent to the Euclidean parallel postulate (which we are allowed to assume on all GRE questions). It should not be two hard to prove this or find a proof of this if you are unconvinced, but it should make sense intuitively because if it were the case that BF > DF, we would necessarily have BF (BE) intersecting CD, which is impossible by assumption CD || BE. Thus it must be the case that BC = DF and BF = CD and since we know BC = CD, we have by transitivity of equality that BF = DF (that is, they are all equal to each other).

As for your second question, we do know that BCDF is a parallelogram because CD || BF and BC || DF. Furthermore, since we know by the above reasoning that CD=DF, if we were to draw a circle of radius CD centered at D, it would intersect both C and F. We can then draw another circle of the same radius centered at B, which will also intersect C and F. You might recall that this is the construction of a perpendicular to BD. Now, we know that AB || DE and that AB=DE. We then have that BD || AE (again by statement equivalent to Euclid's Parallel Postulate), thus CF must be perpendicular with AE. We have CF || DE because both make right angles with AE and both intersect the parallel lines BE and CD. Then by Euclid's Fifth, we have CF=DE.

HI,
As per Euclidean parallel postulate, the distance between two parallel lines at point will always be equal, so yes we can concur that CD =BF and BC = FD, but since BC = CD, we can write as BC = CD = DF = FB,

This led us that BCDF is a rhombus, moreover the diagonal bisect each perpendicularly so CF and BD will be perpendicular and if we consider two triangles BCD and BDF, they will always have equal area.

The only confusion with the statement CF || DE,

However I approached a different way,

SInce AB || DE and BD || AE also AB= DE and BD = AE (Euclidean parallel postulate), so ABDE is a parallelogram and the diagonals of the parallelogram divides it into 4 triangles of equal area.

Critical and C

imagine it like a letter symmetric envelope. very fast method

It's not given that it is a square. May be a square, maybe not. But it's DEFINITELY a RECTANGLE. Join BD to complete the rectangle.

Now, area of triangle is width*length / 2.

To find the area of the rhombus, use BD dissector and calculate the area of top and bottom triangles. The length is l for both. And the height is half of the width because it's a rectangle and the diagonal intersects at the center, dividing the sides into equal proportions. So height = width/2.

So rhombus area = 2 *[(w/2)*(l)]/2 = wl/2.

Hence both areas are equal.


(Note: Ultimately being a square or rectangle does not matter in the end since the diagram is symmetric and can be stretched to form a square OR a rectangle but the PROPORTION of the areas of the 2 quantities will not change.)