posted by kwack .
Find the equation of the tangent line to the curve y=5xcosx at the point (pi,–5pi).
The equation of this tangent line can be written in the form y=mx+b where
y=5x cos x
y'=slope m= 5cosx-5xsinx at x=pi
line equation: y= -5x+b
now to find b, you know the point(PI,-5PI) is on it, so
-5pi--5PI+b so b=0