math problems

1) tan ( A + B )
= tan ( 180* - C )
= - tan C [ Ans : A ]
2) Sub x = 0,
0^2 + y^2 + 3 ( 0 ) + Ey + 9 = 0
y^2 + Ey + 9 = 0
Since the circle touches the y - axis, so delta = 0,
E^2 - 4 ( 1 )( 9 ) = 0
E^2 = 36
E = 6 or -6 [ Ans : A ]

2007-12-03 21:58:50 補充：
For Q1, by angles sum of triangle, A + B + C = 180* and so A + B = 180* - CSince in 2nd quad, tan is -VE, tan ( 180* - C ) = - tan CFor Q2, touching y - axis means touching the line x = 0 and so by substitution, u can obtain y^2 + Ey + 9 = 0 and by the condition: delta = 0, E can be solved.

1. a+b+c=180
a+b=180-c
tan(a+b)=tan(180-c)
tan(a+b)= - tan c

thus.. the answer is (a).

2. because of " touches " this word......
delta=0
it touches the y-axis...........so x=0
y^2+Ey+9=0
delta=0
E^2-4(1)(9)=0
E^2-36=0
E^2=36
so....E=6 or-6

thus.. the answer is (a).