Tuesday, February 7, 2012

How do I find the coordinates of the two points on the curve y=ln(x^2+1) where the gradient is 5/13

To find the coordines of the 2 points on the curve  y =
ln(x^2+1), we  find dy/dx and equate it to 5/13 and solve for x. Then the solution value
of the x will be put in y = ln(x^2+1)to find the y coordinate
value.


dy dx = {ln(x^2+1} = {1/(x^2+1)}( href="mailto:x@+1)'">x^2+1)' = 2x/(x^2+1)


So
 2x/(x^2+1)  = 5/13 and solve for x.


Multply by 
13(x^2+1)


26x =5(x^2+1)


We
write this as a quadratic equation:


5x^2-26x +5=
0.


(5x-1)(x-5) = 0


5x-1 = 0 .
Or x - 5 =0


Or x = 1/5 . Or x=
5.


So to get yhe  corresponding y coordinates we put these 
solution values of x in  y = ln(x^2+1)


When x = 1/5 , y =
ln{(1/5)^2+1) = ln (26/25)


When x = 5, y = ln(5^2+1) =
ln26.


Therefore the slope is 3/13 for the curve y =
ln(x^2+1) at (1/5 , ln(26/25) and  (5 , ln25).

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