Wednesday, 28 May 2014

`y = log_3(x) , (27,3)` Find an equation of the tangent line to the graph of the function at the given point

`y=log_3(x)`


The line is tangent to the graph of the function at (27,3). The equation of the tangent line is _____.


To solve, the slope of the tangent line should be determined. Take note that the slope of a tangent line is equal to the value of the derivative at the point of tangency.


To determine the derivative of the function, apply the formula `d/dx[log_a(u)]=1/(ln(a)*u)*(du)/dx`.


`(dy)/dx = d/dx[log_3 (x)]`


`(dy)/dx =1/(ln(3)*x) * d/(dx)(x)`


`(dy)/dx =1/(ln(3)*x)*1`


...

`y=log_3(x)`


The line is tangent to the graph of the function at (27,3). The equation of the tangent line is _____.


To solve, the slope of the tangent line should be determined. Take note that the slope of a tangent line is equal to the value of the derivative at the point of tangency.


To determine the derivative of the function, apply the formula `d/dx[log_a(u)]=1/(ln(a)*u)*(du)/dx`.


`(dy)/dx = d/dx[log_3 (x)]`


`(dy)/dx =1/(ln(3)*x) * d/(dx)(x)`


`(dy)/dx =1/(ln(3)*x)*1`


`(dy)/dx = 1/(xln(3))`


The point of tangency is (27,3). So plug-in x = 27 to the derivative to get the slope of the tangent.


`m=(dy)/dx = 1/(27ln(3))`


Hence, the line that is tangent to the graph of the function at point (27,3) has a slope of `m = 1/(27ln(3))` .


To get the equation of the line, apply the point-slope form.


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


Plugging in the values of m, x1 and y1, it becomes:


`y - 3= 1/(27ln(3))(x - 27)`


`y-3=1/(27ln(3))*x - 27 * 1/(27ln(3))`


`y - 3 =x/(27ln(3)) - 1/ln(3)`


`y =x/(27ln(3)) - 1/ln(3)+3`



Therefore, the equation of the tangent line is `y =x/(27ln(3)) - 1/ln(3)+3` .

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