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## Comment on

Possible Right Angle## Can we say that since AC = 12

## Unfortunately, that approach

Unfortunately, that approach is not valid. You're assuming that angle ACB is a right angle in order to prove that angle ACB is a right angle.

Here's what I mean. We know that AC = 12 and BC = 5. However, we cannot conclude that the length of side AB is 13, because we don't know for certain that triangle ABC is a right triangle (in fact, the goal of this question is to determine whether or not triangle ABC is a right triangle).

So, we cannot assume that triangle ABC is a right triangle and then use this assumption to conclude that triangle ABC is a right triangle.

## well, we cannot assume that

## That's right; the hypotenuse

That's right; the hypotenuse (AB) can vary, because ∠ACB can vary.

## When evaluating the statments

## We can't say that the

We can't say that the triangles are similar, and we can't say that they're equal. This is because we can't show that the two triangles have any angles in common.

## Hi Brent!

If BC divides AC in 2 equal parts, can't it be the bisector and then ACB be 90°?

## Hi Laura,

Hi Laura,

Since BC divides AC in 2 equal parts, BC COULD be the bisector and ∠ACB COULD be 90°. However, that need not be the case.

IF it were true that AB = BD (making ∆ABD an isosceles triangle), then ∠ACB would definitely be 90°.

However, since we don't know whether AB = BD, we can't say for certain that ∠ACB = 90°.

Does that help?

Cheers,

Brent

## Yes, thank you!

## Hi Brent,

I selected B using the following reasoning. Please help what I missed out on:

Since the AC=CD, then the angle opposite them must also be equal i.e. angle(ABC)=angle(DBC), hence when we move the point B in either direction, the angles do not remain equal.

Warm Regards,

Pritish

## Be careful. If AC = CD, we

Be careful. If AC = CD, we can't necessarily conclude that ∠ABC = ∠DBC.

## Hi Brent,

Doesn't it follow the rule that the angles opposite the sides with the length have the same angle?

Warm Regards,

Pritish

## That rule applies to 2 (of

That rule applies to 2 (of the 3 sides) of a triangle. More here: https://www.gmatprepnow.com/module/gmat-geometry/video/865

In this case, AC and CD are not two sides of the same triangle.

We can say that AC and CD COMBINE to make ONE side of triangle ABD, but this doesn't really help us.

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