Differentiating implicitly to find Clear the fraction by multiplying through by LCD=x When y is the dependent variable and x is the independent variable, the point-slope equation of the line through (x1,y1) with slope m is . But in this case, x is the dependent variable and t is the independent variable, so the equation of the line through (t1,x1) with slope m is . (t1,x1) = (0,1) And the slope m is x' evaluated at this point: So So substituting m = -1, and (t1,x1) = (0,1) Edwin