Help with partial derivatives of trig functions?

PARTIAL DERIVATIVE IS MOST EASY AS COMPARE TO DERIVATIVE.It involve 2 steps Q(1)-at first find d/dx f(x,y)=3cox3x and second step is d/dy f(x,y)=-5sin5y it depend on u what partial derivative do u find.d/dx sinx=cosx, d/dy cosy=-siny. 2)d/dx f(x,y)=e^y,d/dy f(x,y)=e^x.3)d/dx z=cos(x+2y), d/dx(d/dx)=-sin(x+2y), (!!) d/dy z=2cos(x+2y), d/dy(d/dy)z= -4sin(x+2y). soi think your ans is complete. remember you consider variable only those terms which partial derivative w.r.t (say x) you want find ,another variable suppose constant.number of variable in eqs=number of partial term eqs. i always try to solve all question entirely. you submit what think dont solved about Q.i try explain more tips and shorcut method . i am BSC(MATH,PHYSICS).GOOD LUCK