令dy/dx = p,则d²y/dx² = pdp/dy
原方程就变成pdp/dy = -ksiny
dy乘过去,积分得
p²/2 = kcosy + C
dy/dx = p = √(2kcosy + 2C)
dy/√(2kcosy + 2C) = dx
再积一次分就行了.