PP-T1-DS-174

[lucyyeh]

174. 17336-!-item-!-187;#058&011114
In the xy-plane, what is the y-intercept of line l

(1) The slope of line l is 3 times its y-intercept.

(2) The x-intercept of line l is - 1/3.

ANS: E

可是我算C
設l 為y=ax+b
由(2)通過(-1/3, 0)則 -1/3a+b=0
由(1)a=3b
這樣不就算出來了嗎？

唉...我是那邊又算錯呢？
請各位高手多多幫忙，謝謝！

[scottkidd]

174. 17336-!-item-!-187;#058&011114
In the xy-plane, what is the y-intercept of line l

(1) The slope of line l is 3 times its y-intercept.

(2) The x-intercept of line l is - 1/3.

ANS: E

可是我算C
設l 為y=ax+b
由(2)通過(-1/3, 0)則 -1/3a+b=0
==>有這樣的算法嗎？樓主請問這個算法是怎麼來的，
一條線是兩點才連成一直線耶，怎麼一個點就把方程式算出來了？

[lucyyeh]

這一題我(1)這樣沒錯拉，將點帶入就是方程式，一個點只是沒解，不是不能設方程式
我剛剛問人才發現我實在很笨
-1/3+b=0 跟(2)中的a=3b是一樣的
所以這一題是E

[chris8888]

In the xy-plane, what is the y-intercept of line l

(1) The slope of line l is 3 times its y-intercept.
根據 y = mx + c ; 假設y-intercept is (0, a)
代入 得 0 = 3am + c => 3am = -c, 算不出 insufficient
(2) The x-intercept of line l is - 1/3.
(-1/3, 0) => 代入y=mx + c => 0 = -1/3m + c , 光只有這不行 insufficient

(1) + (2).
y=3am + c 代入(-1/3, 0) 得 0 = 1/3am + c => am = -3c ; still insufficient

ANS: E