如何求得一个方程的斜率

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   比如你有(x,y)形式的两点。 即 P1:(x ₁ ,y ₁ ) 和 P2(x ₂ ,y ₂ )。
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   斜率就是竖直变化除以横向变化：就是用坐标上升量除以坐标右移量。 “竖直变化”就是y坐标值的变化（Y轴是竖直方向的），横向变化值，就是坐标右移变化量。（X轴是水平方向的。）
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   因此斜率的等式是 (y ₂ - y ₁ )/(x ₂ - x ₁ )。 可以用希腊字母 “Δ”（“delta”）表示 ，它的意义是 “ d ifference of”（差值）。因此斜率可以表示为 Δy/Δx，意为 "y 坐标变化/ x坐标变化"。
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   任何时候，水平方向线的斜率都是0 。 为什么？这是因为y坐标的变化量为零。因此Δy = 0，则 Δy/Δx = 0。
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   任何时候，竖直方向的线斜率都不存在。 为什么？这是因为x坐标变化量为零。因此 Δx = 0，因此 Δy/Δx 没有实数意义。
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这部分数学比上面的部分要高深的多。如果你还没有上过微积分课，就可能看不懂这部分，也对你没有什么用处。

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   你已经知道，微分方程可以告诉你某点的斜率。

   • 换句话说， f’(x) = 是方程在 (x,f(x))的斜率。
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   把f(x) 整理到等号一边，这样另一边只有常数项和x项了。 然后求微分。
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   想要找出某个点的切线斜率，将x代入 f’(x)。

   • 因此想要找出 x = k 的斜率，带进k，得到 f’(k)。
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