Why can't a term within a polynomial have a negative exponent ?

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Sir/Mam following is my question :

Why does a boxer moves his head backward to minimize the effect of the coming punch ?

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Sir/Mam following is my question :

When a force acting on a body has equal and opposite reaction, then why should the body move at all?

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Sir/Mam following is my question :

A body is moving along a tough level road with a speed of 15m/s among a given direction. Does any force is required to maintain this speed ? Why ?

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Hi Sir/Mam

If x + y = 4

Find the value of x^{​​​​​​3} - 64 + y^{​​​​​​3}

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