數學問題請問點計?

(i)
L: x + 2y - 1 = 0 passes through A(a, 1).
Then, put x = a and y = 1 in L.
a + 2(1) - 1 = 0
a = -1

L passes through B(7, b).
Then, put x = 7 and y = b in L.
7 + 2b - 1 = 0
b = -3

(ii)
A = (-1, 1)
B = (7, -3)
The mid-point of AB is ( (-1 + 7)/2 , (1 - 3)/2 ) = (3, -1)
The slope of AB = The slope of L = -1/2
(Or [1 - (-3)]/(-1 - 7) = 4/(-8) = -1/2)

The perpendicular bisector of A and B passes through (3, -1) and with a slope of -1/(-1/2) = 2

The equation is
y - (-1) = 2(x - 3)
y + 1 = 2x - 6
y = 2x - 7 or 2x - y - 7 = 0