Sunday, October 27, 2013

Find the equation of the line with slope -1 that is the tangent to the curve y=1/(x-1).

To solve, let's consider the given slope of a line.


`m=-1`


Take note that when a line is tangent to the curve, its derivative at the point of tangency is equal to the slope.


`y' = m`


`y'=1`


To determine the point of tangency, take the derivative of the function of the curve.


`y= 1/(x-1)`


`y=(x-1)^(-1)`


`y'=-1(x-1)^(-1-1) * (x-1)'`


`y'=-(x-1)^(-2)*(1)`


`y'=-1/(x-1)^2`


Then, plug-in the value of the derivative.


`-1=-1/(x-1)^2`


`1=1/(x-1)^2`


`(x-1)^2=1`


`x-1=+-1`


`x=+-1+1`


`x_1=0`


`x_2=2`


To solve for the values of y, plug-in x=0 and x=2.


`x_1=0`


`y_1=1/(0-1)=-1`


`x_2=2`


`y_2=1/(2-1)=1`


So, there two points on the curve in which the tangent have a slope of -1. These are:


(0,-1) and (2,1). 


This means that there are two lines that have a slope of -1. To determine the equation of each line, use the point-slope form.


The first line has a slope of -1 and passes the point (0,-1).  So its equation is:


`y - y_1 = m(x - x_1)`


`y - (-1) = -1(x-0)`


`y+1=-x`


`y=-x - 1`


The second line has a slope of -1 and passes the point (2,1). So its equation is:


`y - y_2=m(x- x_2)`


`y-1 = -1(x-2)`


`y-1=-x+2`


`y=-x+3`


Therefore, the equation of the lines with slope -1 and tangent to the given curve are:


`y=-x-1`


and


`y=-x+3` .

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