由題意得：
y? = (1/5)x + 1，y? = (2/4)x + 1 = (1/2)x + 1。
要求 y? = -y?，即：
(1/5)x + 1 = -
解得：
(1/5)x + 1 = -(1/2)x - 1
(1/5 + 1/2)x = -2
(7/10)x = -2
x = -20/7
y? = (1/5)x + 1，y? = (2/4)x + 1 = (1/2)x + 1。
要求 y? = -y?，即：
(1/5)x + 1 = -
解得：
(1/5)x + 1 = -(1/2)x - 1
(1/5 + 1/2)x = -2
(7/10)x = -2
x = -20/7