Isosceles triangle...

[moonlite]

In isosceles Δ RST, what is the measure of ∠S?
(1) The measure of R is 100◦.
(2) The measure of T is 40◦.

OA is C

If it's an isosceles triangle that means 2 sides are equal so why can't it be D? Please explain. Thanks.

[kris610]

I thought it was A. If one angle is 100, the other two must equal 80, each being 40 because the tri is iso.

B is not sufficient as it could be 40 40 100 or 70 70 40.

[krazy800]

Agree with u Kris.. the answer should be A

[4meonly]

I angree with A
100+S+T=180
S+T=80
S=T=40

if R=100 then it can not be on the base

[Natalllllia]

In isosceles Δ RST, what is the measure of ∠S?
(1) The measure of R is 100◦.
(2) The measure of T is 40◦.

OA is C

If it's an isosceles triangle that means 2 sides are equal so why can't it be D? Please explain. Thanks.

I met same problem only with diffrent angles

In isosceles Δ RST, what is the measure of ∠R?
(1) The measure of T is 100◦.
(2) The measure of S is 40◦.

OA : A

[vittalgmat]

Here is my approach..
WKT (we know that) triangle is isoscles.
=> 2 sides are equal and corr angles are equal as well.

stmt 1.
R = 100deg

So the other two angles are equal coz we cannot have two angles = 100 and still the figure be a triangle.

Sufficient.

Stmt 2.
T = 40

The triangle could be 40-40-100 OR 40-80-80 triangle
so insufficient.

so IMO it is A.
moonlite: pls check the OA ??

Takeaway: If u r given an ISOSCLES triangle, and if one of the angles > 60, then u can figure out all the angles of the triangle.

Also if u r given one of the isoscles side, u can predict the third side using 3rd side = DSS (
Difference between two sides < third Side < Sum of two sides)


HT helps.

[nhai2003]

I agree with Vitalgmat,
THis DS in GMATprep I did this morning.
The OA is A