Question

已知a^2-3ab-4b^2=0则(a^2-3ab+b^2)/(ab-a^2+9b^2)的值,

Answer 1

a^2-3ab-4b^2=（a+b)(a-4b)=0,

——》a=-b,或a=4b,

a=-b时,

(a^2-3ab+b^2)/(ab-a^2+9b^2)=5b^2/7b^2=5/7；

a=4b时,

(a^2-3ab+b^2)/(ab-a^2+9b^2)=5b^2/-3b^2=-5/3.