$\"\"$

The general form of equation is $\"\"$ .

The rotation equation is $\"\"$ .

Where $\"\"$ and $\"\"$ .

Find $\"\"$ .

$\"\"$

Apply formula: $\"\"$ .

$\"\"$

$\"\"$ .

Hence,the value of $\"\"$ does not change under a rotation of axes.

Therefore, the value of $\"\"$ invariant.

$\"\"$

$\"\"$ , the value of $\"\"$ invariant.