Mathematical PJ

Once 'Constant' and e^x ( e raised to x) were walking down the road.
Suddenly 'Constant' screamed and said:

"I m going bcoz 'Differential' is coming and if he sees me he will eliminate me."

But e^x stands firm in front of ' Differential' and says:

"U can't do anything with me. I m e^x and will always be e^x."


'Differential starts laughing........
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Why......
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Differential says: " I m not d/dx, I m d/dy."