PP-DS-Test1-189

[chris8888]

189. 18439-!-item-!-187;#058&011934
In the xy-plane, the line k passes through the origin and through the point (a,b), where ab 0. Is b positive?

(1) The slope of line k is negative.
y = Ax + c => slope 是負的
y = -Ax + c 代入(0, 0 )題目說經過 ; c = 0
y = -Ax insufficient

(2) a < b
insufficient

1+2.
y = -Ax <== 代入(a, b)
b = -A * a
=> b/a = -A
=> 由於b > a, 但是相除必須等餘負的A
=> b/a = 負的
=> 根據 b > a的(2)條件
=> b > 0 and a < 0
sufficient

[Philosophia]

這題題號應是 P2-DS-Q189