x(x-1)(x+1)=990
x(x^2-1)=990
x^3-x-990=0
x^3-10x^2+10x^2-100x+99x-990=0
(x-10)(x^2+10x+99)=0
x^2+10x+99
=x^2+10x+25+74
=(x=5)^2+74>0
所以x-10=0
x=10