Cách làm cho bạn:
a) 747 $\notin$ P (747 $\vdots$ 3); 235 $\notin$ P (235 $\vdots$ 5); 97 $\in$ P;
b) a = 835.123 + 318 = 835.41.3 + 106.3 = 3. (835.41 + 106)
$\Rightarrow$ a $\vdots$ 3 $\Rightarrow$ a $\notin$ P.
c) b = 2.5.6 – 2.29 = 2. (5.6 – 29)
$\Rightarrow$ b $\vdots$ 2 $\Rightarrow$ b $\notin$ P.
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