y = (1 - x²)/(1 + x²)
两边同乘以(1 + x²):(1 + x²)y = 1 - x²
y + x²y = 1 - x²
移项:x²y + x² = 1 - y
x²(1 + y) = 1 - y
x² = (1 - y)/(1+ y)
请注意用括号把问题表示清楚,否则很容易有误解.