Examine the function

f(x) = x + (2/x)

Find the point on the curve at which the tangent lines pass through the point (3, 3).

(x,y) = ?

Verified Answer :

The tangent line to the function's graph at any point \((a,b)=\left(a,a+\dfrac2a\right)\) has slope

\(f'(x)=1-\dfrac2{x^2}\)

and thus has the equation

\(y-a-\dfrac2a=\left(1-\dfrac2{a^2}\right)(x-a)\implies y=\left(1-\dfrac2{a^2}\right)x+\dfrac4a\)

Any such line that passes through (3,3) will then satisfy

\(3=\left(1-\dfrac2{a^2}\right)3+\dfrac4a\mplies\dfrac6{a^2}=\dfrac4a\)

\(a\) clearly can't be 0, so we can divide both sides by \(\dfrac1a\) to end up with

\(\dfrac6a=4\implies a=\dfrac64=\dfrac32\)

So the only point that answers the question is \(\left(\dfrac32,\dfrac32+\dfrac2{\frac32}\right)=\left(\dfrac32,\dfrac{17}6\right)\)