Derivative- Normal line（急!!!)

搵normal既話,要先搵slope of tangent
既然我唔知個function係咩,我唯有用F'(x)繼續做落去.

　　　　　　　　　　　　　　y = F(x)
　　　　　　　 slope of tangent = F'(x)
As tangent ⊥ normal,
slope of tangent * slope of normal = -1
　　　　　F'(x) * slope of normal = -1
　　　　　　　slope of normal = -1 / F'(x)

slope of normal line to F(x) at x=4 = -1 / F'(4)

Then, the equation for the normal line to F(x) at x=4 is
　　　　　[ y - F(4) ] / ( x - 4) = -1 / F'(4)
　　　　　F'(4)y - F(4) F'(4) = - x + 4
　　　　　　　　 x + F'(4)y = F(4) F'(4) + 4 //