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Simplify the following expressions if possible.

1)

$\displaystyle{ {r^{4}-3r^{4}}=}$

2)

$\displaystyle{ (r^{4})(r^{4})=}$

3)

$\displaystyle{ {r^{4}+3r^{3}}=}$

4)

$\displaystyle{ (r^{4})(3r^{3})=}$

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