Tuesday, March 17, 2015

Prove that the equation of the tangent line to the graph of function f=e^(x^2)/x, in the point x=1, is y=x*e.

First, let's write the equation of the tangent line to the graph, to see what elements we have and what we have to find out.


y - f(1) = f'(1)*(x-1)


It's obvious that we have to calculate f(1), f'(x) and f'(1).


Let's begin with f(1).


We'll substitute x by 1, in the expression of the fucntion f(x).


f(1)=e^1/1


f(1)=e


Now, we'll calculate the first derivative of the function, using the product rule:


f'(x) = {[e^(x^2)]'*x - e^(x^2)*x'}/x^2


f'(x) = [2x*e^(x^2)*x - e^(x^2)]/x^2


We'll factorize and we'll get:


f'(x) = [e^(x^2)(2x^2 - 1)]/x^2


Now, we'll calculate f'(1).


f'(1) = [e^(1^2)(2 - 1)]/1^2


f'(1) = e


Now, we'll substitute the values for f(1) and f'(1), into equation of the tangent line, to verify if it's expression is the same with the one given into enunciation.


y - f(1) = f'(1)*(x-1)


y - e = e*(x-1)


We'll open the brackets and we'll get:


y - e =e*x - e


We'll reduce the similar terms:


y=e*x   q.e.d.

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