Question

9ab(10a2−b2)+10ab(b2−9a2) при a=10,b=−2.

Answer 1

$\displaystyle\bf\\9ab(10a^{2}-b^{2})+10ab(b^{2} -9a^{2} )=90a^{3} b-9ab^{3}+10ab^{3}-90a^{3} b=\\\\\\=\underbrace{(90a^{3} b-90a^{3} b)}_{0}+\underbrace{(10ab^{3} -9ab^{3} )}_{ab^{3} }=ab^{3} \\\\\\a=10 \ \ ; \ \ b=-2\\\\ab^{3} =10\cdot(-2)^{3} =10\cdot 8=80$