07 HKCEE Maths M.C.Qu. 15&32

15. C
32. ax+by+1=0
when x=0
by+1=0
y=-1/b
Because y-intercept is negative, so b is positive
b>0
when y=0
ax+1=0
x=-1/a
Because x-intercept is negative, so a is positive
a<0

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Q15係C
Q32係A
你先將佢change subject
-ax 1
y= ------- - -------
b b
-1/b係y-int... 因為佢y-int係-ve...所以b要係+ve
-a/b係slope.....睇返個圖...slope係-ve
而b係大過0....a>0先可以令slope變-ve

所以a,b>0

2007-05-07 18:52:37 補充：
y= -ax/b - 1/b