Tricky DATA sufficiency question concept similar to an OG Q

[vak3e]

Hi all, here's a Q that stumped me:

If xy=-8, what is the value of xy(x+y)
1. x-y = 5

I don't understand why 1. would be insufficient to solve this.
If you square xy(x+y)
(xy)^2(x^2 + 2xy + y^2)

Then square x-y
x^2-2xy+y^2 = 25

It's easy to see how we could plug in all the missing pieces and get a value for xy(x+y). I'm guessing there's something wrong with my idea of squaring everything, but as far as I know, it's OK to square stuff w/o hesitation, but taking the square root is what gives you multiple solutions. Can someone please help explain this?! Thank you!

Related subquestion:
If the q had been:

If xy=-6, is xy(x+y)=6
1. x-y = 5

Would this make my approach more valid? If so any ideas why?

Thank you so much guys!

[RonPurewal]

This is the wrong folder for this question.

If you are trying to mock-up an OG question, then you should ...
- Post your question in the general math folder
"- Give the edition # and problem # of the original OG problem (so that we can verify that your problem is different enough to avoid violating the copyright)

Thanks.