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the potential energy is U(x)=[tex]\Sigma[/tex][tex]U_{G}e^{iGx}[/tex]

but why it is also equal to-> U(x)=[tex]\Sigma[/tex][tex]U_{G}e^{iGx}+e^{-iGx}[/tex]

from text explanation, i couldn't get it why there is an extra term of [tex]e^{iGx}[/tex]

any 1 care to enlight me on this? thanks.