Posted by **Help Please** on Friday, March 9, 2012 at 9:30pm.

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

0<=x<=pi/4

0<=y<=pi/4

i have that

dz/dx=cos(x)+cos(x+y)

dz/dy=cos(y)+cos(x+y)

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 -
**Anonymous**, Monday, April 2, 2012 at 2:39pm
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

d=fxx*fyy-fxy^2

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

## Answer this Question

## Related Questions

- TRIG! - Posted by hayden on Monday, February 23, 2009 at 4:05pm. sin^6 x + cos^6...
- tigonometry - expres the following as sums and differences of sines or cosines ...
- Mathematics - Trigonometric Identities - Let y represent theta Prove: 1 + 1/tan^...
- Math - Calculus - The identity below is significant because it relates 3 ...
- algebra - Can someone please help me do this problem? That would be great! ...
- trig - Reduce the following to the sine or cosine of one angle: (i) sin145*cos75...
- Calculus 12th grade (double check my work please) - 1.)Find dy/dx when y= Ln (...
- Trig - Find sin(s+t) and (s-t) if cos(s)= 1/5 and sin(t) = 3/5 and s and t are ...
- calculus - Differentiate. y= (cos x)^x u= cos x du= -sin x dx ln y = ln(cos x)^x...
- Trig - Given: cos u = 3/5; 0 < u < pi/2 cos v = 5/13; 3pi/2 < v < ...

More Related Questions