@Kitonum I am asking this because it can not solve also x^2+y^2=z*t equation as well. Which is strange to me.

@Kitonum Thanks for quick response. Is there any rule which equations can be solved in maple? In the help there is no restriction so one can assume that all 2-nd order simple diophantine equations can be solved, which is obviously not true. So could you please specify the solvable equations set somehow?