\tag is not allowed in eqnarray

Question

I used \tag{} in eqnarray. But is not permitted and have an error "! Package amsmath Error: \tag not allowed here". Is there a solution to this? Please do as necessary after checking.

Answer 1

You asked,

Is there a solution to this?

Short answer: Yes.

Slightly longer answer: Don't use eqnarray environments. Instead, be sure to load the amsmath package and to use align environments.

Incidentally, \tag and \notag are macros that are provided by the amsmath package -- the very same package that defines the align environment. By design, there is no way to use them inside eqnarray environments.