Search All of the Math Forum:
Views expressed in these public forums are not endorsed by
NCTM or The Math Forum.



[apcalculus] Re: 2001 AB6 question
Posted:
Mar 19, 2005 12:30 PM


Hello everyone,
 On Fri, 18 Mar 2005, Shawn J. Godack wrote:
After separating variables and integrating you get
y^(1) = 6x  x^2 + C
If you substitute the point (3,1/4) in now, you get one value for C.
If you solve the equation first for y and get y = 1 / (6x  x^2) + C, this C value will be different.
Which way is correct or
 !!!! should the C value remain the in the denominator? !!!!  Yes, the C must remain in the denominator. Your solution is
1/y = 6x  x^2 + C and when you invert and multiply by
minus one, the whole expression goes to the denominator. The constant remains the same in both cases.
While I don't have the examination, it is clear that the DE involves a y^2 term. Squaring (1/(6xx^2)) + C and (1/(6xx^2 +C)) will give you two entirely different results.
In general, when you obtain a solution that you are not comfortable with, it is worthwhile to check back and see if it fits the original DE.
Hope this helps.
Dick Maher
Richard J. Maher Mathematics and Statistics Loyola University Chicago 6525 N. Sheridan Rd. Chicago, Illinois 60626 17735083565 rjm@math.luc.edu
 You are subscribed to apcalculus as: boxapcalculus@mathforum.org
To unsubscribe send a blank email to leaveapcalculus@lyris.collegeboard.com
To update your preferences, search the archives or post messages online, visit http://lyris.collegeboard.com/cgibin/lyris.pl?site=collegeboard&enter=apcalculus
If you need help using Lyris web interface go to: http://www.lyris.com/lm_help/4.0/WebInterfaceforUsers.html
Visit AP Central(tm)  The online resource for teachers, schools, colleges, and education professionals http://apcentral.collegeboard.com
The College Board 45 Columbus Avenue New York, NY 100236992



