thanks very much for all your help, it now works!

thanks to both of you! i've combined both of your suggestions and can now produce a plot for (u^2/sin(k)^2)-(v^2/cos(k)^2), using the command implicitplot({seq((u^2)/((sin^2)^(k))-(v^2)/((cos^)^2(k)),k=1..3)},u=-1..1,v=-1..1); however i have to have k=1..3 as maple won't let me have k=0..Pi and says it is unable to execute the sequence, any idea what i've done wrong?