Math: Counting Principles and Probability CONFUSION MY GOODNES

Test Preparation SAT Preparation
1

<p>im getting suicidal and depression about math im so sad </p>

<p>so sad</p>

<p>idk wat happend i used to get really good scores on my math like 800 and 770 but now i get crap that has not broken 750 in a long time. im aiming for 800 on this january SAT</p>

<p>IM SO CONFUSED, I THOUGHT I HAD IT UNDER MY BELT BUT I DONT:</p>

<p>So, the question was "Parallelogram with no right angles, what is the probability that two randomly chosen angles are supplementary"</p>

<p>my thought process</p>

<p>o well, 4 angles can be chosen as the first one leaving me with 3 angles left to be chosen for the second one; therefore, there are 12 combinations total, right? err...</p>

<p>so after that, I listed out all the angle combos that add to 180-> 4 combos
4/12-> 1/3</p>

<p>the answer was 2/3, i was confused so i wrote all the possibilities in a table and only came up with 6 combinations... </p>

<p>so what does 4*3 represent? all the combinations and their variations? (ie: AB and BA are counted separately) </p>

<p>im seriously confused... why didn't my way work? </p>

<p>also i was infuriated even further when i got a second question wrong earlier on the same set because i said 4/24I<em>3/24 but it should have been 4/24</em>3/23. i really did not see that and i admit it was not a stupid mistake, it was a mistake i should be really scared about...</p>

<p>im really anxious now... can anybody post a link to some counting and probability excercises?</p>

2

<p>Your mistake is that you should be using a combination and not a permutation. You are counting the total number of ways to choose 2 angles from 4. Once you choose these 2 angles you are NOT arranging them. For example, choosing angle 1 and angle 2 is the same thing as choosing angle 2 and angle 1. So for the denominator simply type 4C2 in your calculator. </p>

<p>It’s also very quick to count these pairs by hand (as I think you’ve noticed). </p>

<p>Sent from my iPhone using [URL=&lt;a href=“Tapatalk”&gt;Tapatalk]Tapatalk[/URL</a>]</p>

3

<p>Another way to think about it: pick an angle. Now, no matter what angle you have picked, of the three remaining angles two are supplementary to the one you picked and the third one is congruent to the one you picked.</p>

4

<p>[Category:Introductory</a> Combinatorics Problems - AoPSWiki](<a href=“http://www.artofproblemsolving.com/Wiki/index.php/Category:Introductory_Combinatorics_Problems]Category:Introductory”>Art of Problem Solving)</p>

<p>For now, I’d suggest looking only at the earlier AHSME or AMC problems (around #15 or earlier), as the rest of them can get quite tricky. Feel free to look at the other problems, but keep in mind the later ones can be fairly difficult.</p>

5

<p>thanks for your help guys…</p>

<p>i’m hoping for the best with this SAT…</p>