The Student Room Group
Reply 1
Ultimate Tutor
can any of u help me solve this question. ive tried n tried but cant seem to reach the answer given by the teacher. the answer is supposed to be 750

this is a table :smile:
________________
x | f(x) | f'(x)
________________
2 | 5 | 10
8 | 7 | -3
________________

find h'(2) if h'(x) = [f(x)]^3

thanks in advance


I would have thought the answer was 125 but I have not used much information from the table.

Can you show your working/thoughts?
Reply 2
em.... y do u say its 125?? [*curious*]

the workin my teacher gave is as below

h'(x) = 3[f(x)]^2 . f'(x)
h'(2) = 3[5^2] . 10 = 750

which i do not understand......
Reply 3
Ultimate Tutor
em.... y do u say its 125?? [*curious*]

the workin my teacher gave is as below

h'(x) = 3[f(x)]^2 . f'(x)
h'(2) = 3[5^2] . 10 = 750

which i do not understand......




I think there was a typing error in the question.

Reply 4
yep that is it. it was perhaps a typing mistake. even so, i do not understand the working. is it a rule or something... i tried googlin but couldnt find anything to my benefit.
Reply 5
google 'Chain rule'
Reply 6
Yeah its a rule you have to learn, I wouldn't worry about it too much, its fairly easy once you remember it.
Reply 7
The working uses the chain rule.
Reply 8
pls help me here..

if its chain rule then its somethign like,
dy/dt = dy/dx . dx/dt
which is in this case..
let y = f(x)
t = h(x) = [f(x)]^3

so dy/dx = f'(x)

dt/dx = 3[f(x)]^2 then dx/dt should be 1/3[f(x)]^2 .... i know im wrong but is this not how it is done..:eek3:

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