\(a)\) \({25^2} - {15^2} = \left( {25 + 15} \right)\left( {25 - 15} \right)\)\( = 40.10 = 400\)
\(b)\) \({87^2} + {73^2} - {27^2} - {13^2}\)
\( = \left( {{{87}^2} - {{13}^2}} \right) \)\(+ \left( {{{73}^2} - {{27}^2}} \right)\)
\(= \left( {87 + 13} \right)\left( {87 - 13} \right) \)\(+ \left( {73 + 27} \right)\left( {73 - 27} \right) \)
\(= 100.74 + 100.46 = 100\left( {74 + 46} \right)\)
\(= 100.120 = 12000 \)
