Question: Problems with the collect command

Please, can someone give me a hand? I can not understand why in the first case the collect command does not work while the second one works correctly. Many thanks for your

esp1 := -a^4+a^2*c^2



esp2 := collect(esp1, a^2);

This, apparently, does not work properly



esp3 := collect(-a^4+a^2*c^2, a^2);



esp11 := -a^2*x^2-a^2*y^2+c^2*x^2



esp12 := collect(esp11, x^2);







Please Wait...