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you change the sign only on the anode because the anode is losing electrons (is the oxidation 1/2 reaction). every cell potential is given in reduction form, so you'll need to switch the sign to account for the oxidation 1/2 reaction.
You change the sign of the reduction potential of the anode/cathode half-reactions based off of the direction of the half-reaction. For example, for Cu2+ + 2e- -> Cu, the standard reduction potential is 0.34 V. But, if this reaction occurs at the anode, then the half-reaction will actually be CU -> Cu2+ + 2e- with a standard reduction potential of -0.34 V. So, you would switch the sign of the reduction potential depending on whether the forward or reverse of the reaction is used.
As stated above, you would change the sign to positive or negative depending on the direction of the reaction from the standard reduction potential. That is why it is also very important to know whether a half reaction is an oxidation or reduction.
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