Example 102

$\;\;\;(9x^{8} + 4y^{9})(9x^{8} - 4y^{9})$

$=(A + B)( A - B) \text{ , where } A = 9x^{8} \text{ and } B = 4y^{9} $

$=A^2-B^2$

$=(9x^{8})^2 - (4y^{9})^2 $

$=81x^{16} - 16y^{18} $