更新时间2019-01-12 11:11:17
原式 = ( 5x^2 + 30xy + 55y^2 )/ ( x^2 + 6xy + 11y^2 ) - 4y^2/ ( x^2 + 6xy + 11y^2 )
= 5 ( x^2 + 6xy + 11y^2 )/ ( x^2 + 6xy + 11y^2 ) - 4y^2/ [ ( x + 3y )^2 + 2y^2 ]
= 5 - 2/ [ ( x + 3y )^2/(2y^2) + 1 ];
显然, ( x + 3y )^2/(2y^2) 越小,2/ [ ( x + 3y )^2/(2y^2) + 1 ] 越大,原式的值越小;
∵ ( x + 3y )^2/(2y^2) ≥ 0, ∴ x = 3y 时, ( x + 3y )^2/(2y^2) = 0,原式 = 5 - 2/1 = 3;
最小值是 3 。
分式=5-(4y²)/[(x+3y)²+2y²],取x=-3y得最小值3