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note that: y 2 e x ( ln ( ( x + y ) y ) e y + ln ( x y 1 ) ) = y 2 e x ( y ln ( ( x + y ) ) e y − y ln ( x ) ) = y 2 e x y ( ln ( x + y ) e y − ln ( x ) ) = y e x ( ln ( x + y ) e y − ln ( x ) ) = y ln ( x + y ) e x e y − ln ( x ) e x = y ln ( x + y ) e x + y − ln ( x ) e x take the limit: y → 0 lim y ln ( x + y ) e x + y − ln ( x ) e x note that using the definition of the derivative, this limit is the same as d x d ( ln ( x ) e x ) = ln ( x ) e x + x e x taking the other limit gives: x → 1 lim ( ln ( x ) e x + x e x ) = ln ( 1 ) e 1 + 1 e 1 = 0 + e = e .