camlan1990
EgmatQuantExpert
camlan1990
Hi Bruel,
For (1): Because (x + y)^2 <= (1^2+1^2)(x^2+y^2) => x^2+y^2 >= 4.5a > 4a. So A is sufficient?
Could you help me find out whether there is any mistake in my solution?
Thanks Bruel,
Dear
camlan1990The highlighted part in your solution above is wrong.
The correct expansion for \((x+y)^2 = x^2 + y^2 + 2xy\).
You on the other hand have wrongly written: \((x+y)^2 = 2(x^2 + y^2)\).
Hope this helped.
Best Regards
Japinder
Dear Japinder,
As I highlighted, (x+y)^2 is smaller or equal 2(x^2 + y^2)
Oops
camlan1990, that was my bad! I didn't notice the '<' sign in the "<="
Now I do see how you got to x^2+y^2 >= 4.5a
But please note that x = 0, y = 0 and a = 0 is one set of values that satisfies this inequality. For these values of x, y and a, the answer to the question 'Is x^2 + y^2 > 4a' is NO
For all other values of x, y and a, the answer will be YES.
Since we are not able to rule out x, y, a = 0, we cannot infer a unique answer to the posed question using St. 1 alone. So, St. 1 is not sufficient.
Hope this helped.
Japinder