one problem that is annoying me!!

Test Preparation SAT Subject Test Preparation
1

<p>i remember coming back to this problem and working on it for like 5 minutes during the november 3rd SAT II Math II...</p>

<p>if x + y = 8
and xy = 20
what is x^2 + y^2 ?</p>

<p>not that one problem will make or break my grade on the test, it just is annoying me that i can't figure it out!!</p>

2

<p>Consider (x+y)^2. This equals 8^2 = 64. Thus 64 = (x^2+y^2)+2xy. But xy = 20. Thus x^2+y^2 = 64-40 = 24.</p>

3

<p>I'm pretty sure it was x-y = 8..</p>

4

<p>it was x-y = 8
xy = 20</p>

<p>x= 10
y= 2</p>

<p>x^2 + y^2 = 104</p>

<p>bada bing bada boom</p>

5

<p>x-y = 8 can be solved the same way with the expansion.</p>

<p>64 = (x-y)^2 = (x^2+y^2)-2xy = (x^2+y^2)-40. So x^2 +y^2 = 104.</p>

6

<p>I love your username avantgardener.</p>

7

<p>OH x MINUS y... haha. anyway its one of those problems where i learned how to do it years ago, but my algebra is super rusty now... eh whatevs. thanks for you explanations! it makes sense now!</p>

<p>and thanks dane :)</p>

8

<p>God I am so *<strong><em>ing *</em></strong>ed at myself right now, I'm pretty sure I marked 84 even though i did the problem just like you guys did. Somehow, I said, "Yeah, 64, then I'll need to add 2xy to cancel out the -2xy, must be 84"</p>

<p>Goddamnit. I hate tests.</p>