<p>If f: (x,y)=(x-y,x+y) for every pair (x,y) in the plane, for what points (x,y) is it true that (x,y)= (x,y)
a The set of points (x,y) such that x=0
b The set of points (x,y) such that y=0
c The set of points (x,y) such that x=y
d The set of points (x,y) such that 2x=y</p>
<p>Besides can someone clarify this f: (x,y) thing to me.</p>
<p>If f: (x,y)=(x-y,x+y) for every pair (x,y) in the plane, for what points (x,y) is it true that (x,y)= (x,y)
a The set of points (x,y) such that x=0
b The set of points (x,y) such that y=0
c The set of points (x,y) such that x=y
d The set of points (x,y) such that 2x=y</p>
<p>what you do is plug in points for each letter and do process of elimination.</p>
<p>for c, it says (x = y) so let's plug in (1,1). plugging in (1,1) = (x-y, x + y), which = ((1-1), (1-1), which equals (0, 0)</p>
<p>for a, plug in (0,1), for b plug in (1,0), and for d plug in (2,4)</p>
<p>get it?</p>
<p>Can u clarify f: (x,y) thing. Is it the same as f(x)=y</p>
<p>Here's non-plug & chug explanation:</p>
<p>Basically, the question says that the function f takes any point (x,y) and makes it (x-y, x+y). If you want the output of (x,y) to be (x,y), x-y must equal x and x+y must equal y.</p>
<p>Thus, x-y = x
y = 0</p>
<p>x+y = y
x = 0</p>
<p>Uh? I think the answer should be the point (0,0) which isn't one of the answer choices. Someone point out my mistake?</p>
<p>EDIT: It's not really f(x)=y. f is just some arbitrary function that acts on both (x,y) and changes them.</p>
<p>what happened to the 5 choice. i thought math II had 5 choices</p>
<p>I get it as (0,0) as well. Anyone have the right answer?</p>
<p>Uh found the answer. Scroll to the bottom of the page, last one. But take the point (1,1), it doesn't fit! I mean, (1-1,1+1), ie. (x-y, x+y) is (0,2) != (1,1), ie. (x,y)!</p>
<p>SparkNotes:</a> SAT Subject Test: Math Level 2: Explanations</p>