1. The problem statement, all variables and given/known data Find all tangent lines of the graph f(x)=x+3/x that have a y intercept of 4. 2. Relevant equations 3. The attempt at a solution Assume a is the x coordinate of a point of tangency. Thus the point of tangency is (a, a+3/a). We know the tangent line must pass through (0,4) so the slope of the line must be (a+3/a-4)/(a-0). f'(x)=1-3/(x^2) Derivative of f at point a must equal the slope of the tangent line, i.e. we must have f'(a)=1-3/(a^2)=(a+3/a-4)/(a-0)=m Solving I get a=3/2. However, looking at the graph of f(x), it seems there should be two points where the tangent line passes through 4, the other one being on the part of the f(x) where x<0. Where did I go wrong?