# Diffrentiation help needed!!

• Nov 26th 2008, 11:47 AM
cougar
Diffrentiation help needed!!
1. A curve C has equation y=x^3 - 5x^2 + 5x +2
a) find dy/dx in terms of x
b)The points P and Q lie on C. The gradient of C at both p and q is 2. The x coordiante of P is 3.
i)Find the x coordinate of Q
ii)Find an equation for the tangest to C at P, giving your answer in the form of y =mx + c
iii)If this tangest intersects the coordinates axes at the points R and S find the length of RS giving your answer as a surd.
• Nov 26th 2008, 01:19 PM
cougar
Sigh im off in a hour...noone can every solve my questions xD
BTW i worked out part a) its 3x^2-10x+5
• Nov 26th 2008, 01:57 PM
AshleyT
Quote:

Originally Posted by cougar
Sigh im off in a hour...noone can every solve my questions xD
BTW i worked out part a) its 3x^2-10x+5

Heh, i don't think you should post here and people do your homework for you whilst you go off its a help forum not a 'do my homework for me forum'.

I take it your doing core 1 in AS levels? Its a simple question that always comes up in an exam and its always the same :D...and their always about 7 marks! An easy 7 marks to bag :D....so please don't think im having a moan :D, i mean it in the best intentions(i did this exam last year) that you need to be able to do it for da exam :D.

Also i'll recommend http://www.mathhelpforum.com/math-he...ths.co.uk....i signed up for it last January and since i have been using it got over 85% in all exams afterwards...just a suggestion...I managed to get both an A in maths and further maths last year and livemaths.co.uk played a really big part in helping me do it, especially because i had some hard family situations going on.

Your correct in that part a is
$
3x^2 - 10x + 5
$

Part i'll work out and post in a second but i'll give you a hint...or are you stuck on something specific?

b.) you need to find when the gradient is 2...so do

$3x^2 - 10x + 5 = 2$
and solve it. You already know one root is 3...because it tells you, so the other answer you get will be the x co-ord to Q.

c.) The tangent...you need to work out the co-ordinate of of P...you already know x is 3(so u can get the y value)...you know the gradient is 2...so use the formular
y - y1 = m(x - x1)
Substitute in your values and re-arrange to get y = mx + c

d.) Now you have tangent etc...draw it and all the points to help you work out what the question is asking.
You can get R and S by putting using the equation of the tangent...subbing in x as 0 (to get where it crosses the y axis) and by subbing in y is 0 (to get where it crosses the x axis)...you now have points s and r and can work out its length by Pythagoras :)

Give it a go...if you still cant get the answer let me know and ill post the working for you :)...in addition feel free to pm me since i've done furthermaths AS already and should be able to help you with D1, C1, C2, M1, FP1, S1.