Can anybody do the following problem?

I don't know how to go about doing it...

Give an example to show that if R and S are both n-ary relations, then

$\displaystyle

P_{i_1,i_2,...,i_m}(R \cap S)$ may be different from $\displaystyle

P_{i_1,i_2,...,i_m}(R) \cap P_{i_1,i_2,...,i_m}(S) $