Hello friends
PLEASE HELP 

I have the following issue 

I want to make one plot with specific values of argument 

here is an example of 2d plot ,

what i want is to plot the same graph only with specific values of x for example x =-pi..pi/2  , -pi/4..pi/4  and so on 

but this has to be in one plot , could please tell me the options how I can manage this problem .  

At the end its a 3Dplot , x is a function of y and t  

so actually i want to make 3d plot in a different ares of y and t 

something like this plot3d(u ,x=-1..1,t=-1..1 , x=2..3 , t=2..3 .... ) 
or maybe in opposite way ,to restrict some values of x and t 

( idea is that function is periodic but I dont want to get crazy high values of the function, especially where functuon goes to infinity  ) 

PLEASE HELP 

thank you very much in advance 

Hello everyone 

I have the following issue :

I want to create a different plots (2d/3d) with varying variables ( I mean expression contains different variables and I am changing their values in order to get different results ) 

for example: 

u=a*x^2+b*y^2

a=1

b=1

plot3d(u, x = -2 .. 2, y = -2 .. 2)

and I am getting an error 

could you please tell me how to plot in that way , that first i define the function with unknown variables , then i am giving them different values ( idea is that i have lots of variables and lots of ranges so it will not be convenient to change them manually into the expression ) and at the end to have a plot 

Thank you very  very much