R+2ｒ+（r^2/R）
微分したらどうなる？
つーか、どうすれば、1-（r^2/R^2）になるんだ？
고민일
R+2 r+(r^2/R)
미분 하면 어떻게 된다？
개-인가, 어떻게 하면,1-(r^2/R^2)가 된다？
悩み事
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