[求助]

husband

161. 16120-!-item-!-187;#058&010949
In the xy-coordinate plane, line m and line k intersect at the point (4, 3). Is the product of their slopes negative?

(1) The product of the x-intercepts of lines m and k is positive.

(2) The product of the y-intercepts of lines m and k is negative.

Ans:C

請問大大,這題要如何推出答案?

macelino

题目问：两条线的斜率乘积是否为负？也就是问两个斜率是否异号．

条件1.m和k在x轴的截距的乘积为正,那么m和k在x轴的截距要么同时为正,要么同时为负.但不足以推出斜率,不充分

条件2.m和k在y轴的截距的乘积为负,那么m和k在y轴的截距必为一正一负,也不足以推出斜率,不充分

两条件一起考虑.那么m和k在x轴的截距要么同时为正,要么同时为负.在y轴的截距必为一正一负,很显然两线斜率乘积异号.充分

C

husband

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