Mathematics
OpenStudy (anonymous):

Derivative of (3/(5t)-1)/((2/t^2)+7)? Answer in the book is: (-21t^2 - 20t + 6)/(5(2 + 7t^2)^2 I don't know what is going on with my math, but I can't seem to get this right :[ Here are my fail attempts : 1. http://puu.sh/4FyMW.JPG 2. http://puu.sh/4FyTA.JPG

hartnn (hartnn):

you cannot multiply and divide by different quantities (5t and t^2) start by multiplying and dividing by 5t^2

OpenStudy (anonymous):

yeah i knew that, but i tried it anyways to see what would happen haha. but in the 1st picture, i felt like i was on the right track, but i don't know anymore

hartnn (hartnn):

in 1st pic , you took the derivative of numerator and denominator separately ? instead of using quotient rule ? thats not allowed

OpenStudy (anonymous):

yes, don't you need the derivative of the numerator and denominator to use the quotient rule?

hartnn (hartnn):

let me check again

OpenStudy (anonymous):

(3/5*t^(-1)-1)/(2t^(-2)+7) d(u/v)/dt=(u'v-v'u)/v^2 u'v=-1(3/5)t^(-2)*v uv'=-4t^(-3)*u That should get you started.

OpenStudy (anonymous):

mmk i'm gonna try that

hartnn (hartnn):

easier thing to do is to multiply and divide by 5t^2

OpenStudy (anonymous):

i'm making this so complicated for myself :( i'll try what you said by multiplying and dividing by 5t^2 instead

OpenStudy (anonymous):

sorry that took so long. for some reason, the 5's cancelled out at the end

OpenStudy (anonymous):
OpenStudy (anonymous):

oh nvm, i see the problem. oops~ thx guys :D

hartnn (hartnn):

oh, so you got it now ?

OpenStudy (anonymous):

yeah. too tired to think at this time. i'm not noticing all these little things

hartnn (hartnn):

$$(10+35t^2)=5^2(2+7t^2)^2$$

hartnn (hartnn):

thats where the error lies

hartnn (hartnn):

you have only written 5(....)^2

OpenStudy (anonymous):

eheh. can't believe i spent 41 mins on a one math problems :(

hartnn (hartnn):

if its any consolation, i have, many times spent 7-8 hours on one problem :P

OpenStudy (anonymous):

lol xD. good night! i think i need it

hartnn (hartnn):

sure, have a sound sleep and sweet dreams :)