x.y=1?
追答注意,这就正如 2ab,也是二次;
xy = 1 就是二次方程了,
要么,解个方程组试一试,
x + y = 2.5
xy = 1
移项,得
y = 2.5 - x
消元,则
x( 2.5 - x ) = 1
2.5x - x" = 1
0 = x" - 2.5x + 1
怎么样?这就变成一元二次方程了吧
x" - ( 5/2 )x + ( 5/4 )" - 25/16 + 16/16 = 0
( x - 5/4 )" - 9/16 = 0
( x - 5/4 )" - ( 3/4 )" = 0
( x - 5/4 - 3/4 )( x - 5/4 + 3/4 ) = 0
( x - 8/4 )( x - 2/4 ) = 0
( x - 2 )( x - 1/2 ) = 0
解方程
x1 = 2,x2 = 1/2,
对应 y 值,就是
x1 = 2,y1 = 1/2;
x2 = 1/2,y2 = 2;
或者用韦达定理,
根据一元二次方程根与系数的关系,
z" - ( x + y )z + xy = 0
z" - 2.5z + 1 = 0
z" - (5/2)z + 1 = 0
同样,
( z - 2 )( z - 1/2 ) = 0
解方程
z1 = 2,z2 = 1/2;
对应 x 与 y 的值,就有
x1 = 2,y1 = 1/2;
x2 = 1/2,y2 = 2;
解方程组用到了一元二次方程,
原来这个 xy = 1 还是一次方程吗?不是啊