I have this expression:

(a*x^2+b*x*sin(y)+c*sin(y))^2+(a*sin(x)^2+b*x)^3

I have to rewrite this as a multipolynomial in the variables x and sin(y).

I'm trying this:

expand((a*x^2+b*x*sin(y)+c*sin(y))^2+(a*sin(x)^2+b*x)^3, x,sin(y));

but i get the same result as expand without the 2 variables at the end.

am i doing it right?

Thanks in advance!

I have a new one :) I need to find the critical points and tell their nature of a 3 variable function. It's a new thing for me and i'm not sure if i did it ok, here's what i have tried:

g := 1-2*x+3*x^2-x*y+x*z-z^2+4*z+y^2+2*y*z;

ex := extrema(g, {}, {x, y, z}, 't'); - stores the extreme value of f in ex

evalf(ex); - displays the extreme value in decimal

t; - contains the critical points of f

evalf(t); - displays the critical points in decimal

Here's what i have to do. i have a system formed of 3 equations, and i have to find the solutions ( there may be only one ), and the make sinus out of it evaluating it with 5 digits.

I am finding problems with plot first: