Rút gọn biểu thức P=(3−x)^2+(x+3)^2−2(x+3)(x−3).

\[P = {\left( {3-x} \right)^2} + {\left( {x + 3} \right)^2}-2\left( {x + 3} \right)\left( {x-3} \right)\]

\[ = {\left( {x + 3} \right)^2}-2\left( {x + 3} \right)\left( {x-3} \right) + {\left( {x-3} \right)^2}\]

\[ = {\left[ {\left( {x + 3} \right)-\left( {x--3} \right)} \right]^2}\]

\[ = {\left[ {x + 3-x + 3} \right]^2} = {6^2} = 36.\]