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Notice that when t=0, they are not always equal. &[Wolfram Notebook][1] [1]: https://www.wolframcloud.com/obj/288dc419-180a-44dc-af09-57ff67bbf186
...and what about an additional variable? exp[x_, y_, z_, u_] := f[x, y, z] + u*g[x, y, z] exp[x, y, z, x] exp[x, y, z, 0]