y = kx+b
0 = k+b
b = -k
y = kx - k
x²/9 - y²/4 = 1
x²/9 - (kx-k)²/4 = 1
4x² - 9k²(x²-2x+1) = 36
(4 - 9k²) x² + 18k²x - 9k² - 36 = 0
Δ = 324k^4 + 4(4 - 9k²)(9k² + 36) = 576 -1152k² = 0
k² = 1/2
k = ±√2 / 2
过点 (1,0) 且与双曲线 x²/9 - y²/4 = 1 只有一个公共点的直线有两条
y = √2 / 2 (x - 1) 或 y = -√2 / 2 (x -1)