Có: \(P=\left\{1;4;9;16;25;36;49;64;81\right\}\)
Ta thấy: \(1=1^2\)
\(4=2^2\)
\(9=3^2\)
\(.......\)
\(81=9^2\)
\(\Rightarrow P=\left\{x|x\in N=k^2\left(k\in\left\{1;2;3;...;9\right\}\right)\right\}\)
P={x\(\in\)N\(|\) \(1\le x\le9\) và \(x^2\)}
P = \(\left\{x\in N,x=k^2,k< 10\right\}\)