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find max, min and saddle points of the give function f(x,y)=sin(x)+sin(y)+sin(x+y)

i have that

and i set them equal to zero but im kinda confused on how to really solve that. i mean i got an answer but that was just from plugging things in and seeing if they worked but i want to know if i can actually solve this without doing that. do i use a trig identity? cause i did that and it came out pretty ugly.

  • calculus -

    Your going to need fxx, fyy, and fxy too. For your critical values of the dz's, you will need to change the cos(x+y) to their proper trig identity because you cant solve for the CV with what you have

    if d=0, then its a saddle point
    if d>0 and so is fxx, then it is a min value
    if D>0, but fxx is <0, then its a max value

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