Is triangle ABC isosceles?
(1) X does not equal Y --> angles at the side BC are not equal --> , but the third side, BC, can be equal to either of them. Not sufficient.
Tells us nothing. X ≠ Y but the third angle could equal x or y in which case the triangle would be isosceles or the third angle would = w in which case the triangle would not be isosceles. Insufficient.
The length of AB is twice the length of BC but we don't know anything about side AC. Insufficient.
1+2) The only way for this triangle to be isosceles is if a.) AC = BC or b.) AB = AC. Neither of these outcomes are possible. If AC = AB then a triangle wouldn't be possible. In any triangle, the sum of two legs must be greater than the third. If AC = BC then the formula for the triangle would be 2BC = BC + BC which isn't possible. We also know that AC ≠ AB because the angles opposite them, x and y respectively, do not equal one another. Thus, AC cannot = AB and AC is a different length than AB. This triangle CANNOT be isosceles. SUFFICIENT.