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I don't understand OA on this one.

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I am getting B as answer...let me know if that is right i would then explain

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gmat2me2 wrote:

I am getting B as answer...let me know if that is right i would then explain

B = 90 degree

Angle ACB=60 degree, and angle CAB=30 degree,

Hence, the triangle must not be isosceles triangle.

A seems insufficient

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gmat2me2 wrote:

I am getting B as answer...let me know if that is right i would then explain

Go for B

angle B = 90 degree

Angle ACB=60 degree, and angle CAB=30 degree,

Hence, the triangle must not be isosceles triangle.

(1) seems insufficient

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B will disporve its not an isoceles triangle
so answer = B

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It has to be B because the second stmt shows that the triangle abc is not isoceles triangle.

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Another vote for B.

In A , the angles of triangle ABC are not known to be equal or not.

In B, the angles are known and are 30,60

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mirhaque wrote:

I don't understand OA on this one.

1 is suff too.

acb=2adc.....1

acb=cad+adc.....2 (external angle equal two other internal angle)

combining 1 & 2 adc=cad.

that means cd=ad.

This goes to prove that triangle abc is not isosceles

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mirhaque wrote:

mirhaque wrote:

I don't understand OA on this one.

1 is suff too.

acb=2adc.....1

acb=cad+adc.....2 (external angle equal two other internal angle)

combining 1 & 2 adc=cad.

that means cd=ad.

This goes to prove that triangle abc is not isosceles

I dont think you can use that theorem here simply because angle A is shared between the triangles .

So your (2) in your explanation is a a doubt for me.

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mirhaque wrote:

mirhaque wrote:

I don't understand OA on this one.

1 is suff too.

acb=2adc.....1

acb=cad+adc.....2 (external angle equal two other internal angle)

combining 1 & 2 adc=cad.

that means cd=ad.

This goes to prove that triangle abc is not isosceles

I get your point now mirhaque. the external angle of ADC is angle B and that should be suffiecient enough to say that A is also sufficient.....

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gmat2me2 wrote:

mirhaque wrote:

mirhaque wrote:

I don't understand OA on this one.

1 is suff too.

acb=2adc.....1

acb=cad+adc.....2 (external angle equal two other internal angle)

combining 1 & 2 adc=cad.

that means cd=ad.

This goes to prove that triangle abc is not isosceles

I get your point now mirhaque. the external angle of ADC is angle B and that should be suffiecient enough to say that A is also sufficient.....

A is not sufficient.

I don't understand how you guys got A sufficient. B should be the answer.

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ashkg wrote:

gmat2me2 wrote:

mirhaque wrote:

mirhaque wrote:

I don't understand OA on this one.

1 is suff too.

acb=2adc.....1

acb=cad+adc.....2 (external angle equal two other internal angle)

combining 1 & 2 adc=cad.

that means cd=ad.

This goes to prove that triangle abc is not isosceles

I get your point now mirhaque. the external angle of ADC is angle B and that should be suffiecient enough to say that A is also sufficient.....

A is not sufficient.

I don't understand how you guys got A sufficient. B should be the answer.

Well the theorem the external angle of a triangle is equal to the sum of opposite angles is used here.

let acb = 2x while ADC = x

since ACB=2x ==>> ACD=180-2x

In triangle ACD angles A + D +C =180

So we have C =180-2x D =x So angle A for ACD has to be x.

So it is isoceles

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gmat2me2 wrote:

ashkg wrote:

gmat2me2 wrote:

mirhaque wrote:

mirhaque wrote:

I don't understand OA on this one.

1 is suff too.

acb=2adc.....1

acb=cad+adc.....2 (external angle equal two other internal angle)

combining 1 & 2 adc=cad.

that means cd=ad.

This goes to prove that triangle abc is not isosceles

I get your point now mirhaque. the external angle of ADC is angle B and that should be suffiecient enough to say that A is also sufficient.....

A is not sufficient.

I don't understand how you guys got A sufficient. B should be the answer.

Well the theorem the external angle of a triangle is equal to the sum of opposite angles is used here.

let acb = 2x while ADC = x

since ACB=2x ==>> ACD=180-2x

In triangle ACD angles A + D +C =180

So we have C =180-2x D =x So angle A for ACD has to be x.

So it is isoceles

The question is asking if trianlge ABC isosceles and not ACD.

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Looks I have drifted to the other triangle :-D
Anyway ...As per the question B would be enough.......Thanks ashkg.....

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For 1), if you draw a isoceles triangle with B a right angle and BA=BC, then you draw a nother isoceles triangle where AC=AD, then 1) would be satisfied. However you can also draw a right triangle ABC where BA<>BC, and then still draw a triangle where AC=AD to satisfy 1). In other words 1) is not sufficient to determine whether ABC is isoleles.

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