i have problems with limit and derivative

first problem :

second oneCode:l i m Cos X - Cos(X+h) h-->0 --------------------- h

third one :Code:find dy/dx : $\displaystyle X^2+3xy+2y^2 = 9$

Code:find points on curve y=tan(x/2) , x belong to ]0,360[ and the tangent is perpendicular to straight line 3x=8-3y i got 4 solutions in this problem .... 2 of them i think refused