黄博士网: 教育网, AI数学手册计算器软件，电化学虚拟实验室，虚拟电化学工作站，电化学软件

直角三角形射影定理（又叫欧几里德(Euclid)定理）：直角三角形中，斜边上的高是两直角边在斜边上射影的比例中项。每一条直角边是这条直角边在斜边上的射影和斜边的比例中项。公式Rt△ABC中，∠BAC=90°，AD是斜边BC上的高，则有射影定理如下：

$\begin{array}{*{20}c} {1.} & {AD^2 = BD \cdot DC} \\ {2.} & {AB^2 = BD \cdot BC} \\ {3.} & {AC^2 = CD \cdot BC} \\ {4.} & {AB \cdot AC = BC \cdot AD} \\ \end{array}$

直角三角形射影定理的证明

在△BAD与△BCD中，∵∠BDA=∠BDC=90°，且∠DBC+∠C=90°， ∴∠A=∠C，又∵∠BDA=∠BDC=90，∴△BAD∽△CBD，∴ AD/BD=BD/CD，即BD²=AD·DC．其余同理可得可证．

任意三角形射影定理

任意三角形射影定理又称“第一余弦定理”：△ABC的三边是a、b、c，它们所对的角分别是A、B、C，则有

$\begin{array}{l} a = b \cdot \cos C + c \cdot \cos B \\ b = c \cdot \cos A + a \cdot \cos C \\ c = a \cdot \cos B + b \cdot \cos A \\ \end{array}$ 由三角形的边角关系可以容易证明该定理

任意三角形射影定理与余弦定理的关系

将任意三角形射影定理的三条公式作如下处理：

$\left\{ \begin{array}{l} a^2 = ab \cdot \cos C + ac \cdot \cos B \\ b^2 = bc \cdot \cos A + ab \cdot \cos C \\ c^2 = ac \cdot \cos B + bc \cdot \cos A \\ \end{array} \right.$ 从上述三条等式中任取两条相加然后减去第三条等式：

$\left\{ \begin{array}{l} a^2 + b^2 - c^2 = 2ab \cdot \cos C \\ b^2 + c^2 - a^2 = 2bc \cdot \cos A \\ c^2 + a^2 - b^2 = 2ac \cdot \cos B \\ \end{array} \right.$ 进一步处理，即可得到任意三角形余弦定理。

面积射影定理

平面图形射影面积等于被射影图形的面积S乘以该图形所在平面与射影面所夹角的余弦。

cosθ=S_射影/S_原（平面多边形及其射影的面积分别是S原,S射影,它们所在平面所成锐二面角的为θ。）

问题？

$(d^0.5y)/dx^0.5 = sin(x-1)sin(y-1)$ == ? $(d^0.5y)/dx^0.5 -cosh(y)-sinh(y)=0$ == ? $(d^1.6y)/(dx^1.6)-int y(x) (dx)^(0.8)-y-exp(x)=0$ == ? $int y(x) (dx)^0.5 -y-exp(x)$ =0 == ? $(d^0.5y)/dx^0.5-exp(y)x=0$ == ? $(d^0.5y)/dx^0.5-exp(y)y=0$ == ? $(d^0.5y)/dx^0.5=cos(x)/xy$ == ? $y(dy^0.5)/dx^0.5-sqrt(x)-1=0$ == ? $(d^1.2y)/(dx^1.2)-2(d^0.6y)/dx^0.6+y-exp(x)=0$ == ? $(d^0.5y)/dx^0.5=cos(y)exp(x)x$ == ? $(d^1.6y)/(dx^1.6)-2(d^0.8y)/dx^0.8+y-exp(x)=0$ == ? $(d^0.5y)/dx^0.5-exp(y)sqrt(x)=0$ == ? $(d^1.6y)/(dx^1.6)-3 (d^0.8y)/dx^0.8+2y-exp(x)=0$ == ? $(d^0.5y)/dx^0.5$ +log(y-1)-exp(x)-x=0 == ? $(d^0.5y)/dx^0.5-exp(y)sin(x)=0$ == ? $(d^0.5y)/dx^0.5 = ysin(x)/x$ == ? $y^((0.5))(x) -4 exp(x)y-exp(x)=0$ == ? $(dy^0.5)/dx^0.5 = 1/(x-y)$ == ? $dy/dx-(d^0.5y)/dx^0.5$ - y - exp(x)=0 == ? $(dy)/dx -exp(y-1)-x-x^2=0$ == ? $(d^1.2y)/(dx^1.2)-3dy^0.6/dx^0.6+2y-exp(x)=0$ == ? $dy/dx-(d^0.5y)/dx^0.5-y-1$ =0 == ? $(d^0.5y)/dx^0.5-cos(y)sin(x)=0$ == ? $(d^1.6y)/(dx^1.6)-(d^0.8y)/dx^0.8-y-exp(4x)=0$ == ? $dy/dx-exp(y-1)-exp(x)=0$ == ? $(dy)/dx - 2(d^0.5y)/dx^0.5-y-exp(x)=0$ == ? $(d^1.6y)/(dx^1.6)-(d^0.8y)/dx^0.8-y-exp(x)=0$ == ? $(d^0.5y)/dx^0.5 -e^(4x)-y$ =0 == ? $y^((0.5))(x) - exp(x)y-exp(x)=0$ == ? $y^((0.5))(x) - exp(x)y-4exp(x)=0$ == ? $(dy)/dx -3(d^0.5y)/dx^0.5 +2y-exp(x)=0$ == ? $y(d^0.5y)/dx^0.5-sqrt(x)-1=0$ == ? $y^((1))(x)-exp(y-1)-x=0$ == ? $(d^1.6y)/(dx^1.6)-(d^0.8y)/dx^0.8-2y-exp(x)=0$ == ? $(d^1.6y)/(dx^1.6)-(d^0.8y)/dx^0.8-y-exp(4x)=0$ == ? $(d^0.5y)/dx^0.5 - log(y-1) - exp(x) + x=0$ == ? $(dy)/dx +asin(y-1) - cos(x)-x=0$ == ? $(d^1.6y)/(dx^1.6)-3(d^0.8y)/dx^0.8+2y-exp(x)=0$ == ? $(dy)/(dx) -sqrt(y-1)-x-1 =0$ == ? $(dy)/(dx) -exp(y-1)-exp(x) = 0$ == ? $(dy)/dx$ +asinh(y-1)-cosh(x)-x =0 == ? $((d^(1/2)y)/dx^(1/2))^2 -3y (dy^0.5)/dx^0.5 + 2y^2 = 0$ == ? $(dy^0.5)/dx^0.5 = cos(x)cos(y-1)$ == ? $(d^0.5y)/dx^0.5 +log(y-1)-exp(x)-x=0$ == ? $(dy^0.5)/dx^0.5 = sin(x-1)exp(y-1)$ == ? $y(d^2y)/dx^2-(dy/dx)^2+1=0$ == ? $y^((1))(x)-exp(y-1)-log(x)=0$ == ? $(d^2y)/dx^2 exp(x)- exp(y-1)=0$ == ? $(d^1.6y)/(dx^1.6)-2 (d^0.8y)/dx^0.8-y-exp(x)=0$ == ? $(d^1.6y)/(dx^1.6)-2 (d^0.8y)/dx^0.8+y-exp(x)=0$ == ? $(dy)/dx -3 (d^0.5y)/dx^0.5+2y-exp(x)=0$ == ? $y^((0.5))(x) - xy-x=0$ == ? $y(dy^3)/dx^3-x^3-3x^2-3x-1=0$ == ? $y^((1.8))(x)-2y^((0.9))(x) +y-1=0$ == ? $y^((0.5))(x)=1/(xy-1)$ == ? $y^((2))(x)y^2-x^2-2x-1=0$ == ? $((d^0.5y)/dx^0.5)^2 -5(d^0.5y)/dx^0.5 +6=0$ == ? $y^((0.5))(x) -2 exp(x)y-4exp(x)=0$ == ? $(d^1.6y)/(dx^1.6)-(d^0.8y)/dx^0.8-y-exp(x)=0$ == ? $y^(0.5)(x)=2yexp(x)$ == ? $y^((0.5))(x)-exp(x)y^2=0$ == ? $(d^1.6y)/(dx^1.6)-2(d^0.8y)/dx^0.8+y-exp(x)=0$ == ? $y^((1))(x)-y^2-xy=0$ == ? $y^((1))(x)-y^((0.5))(x) -y-1=0$ == ? $y^((2))(x) -y^2-x^2=0$ == ? $y^((2))(x) -y^2-x^2-2xy=0$ == ? $y^((0.5))(x) -int y(x) (dx)^0.5-y-exp(x)=0$ == ? $d^0.5/dx^0.5 y -2cos(y)exp(x)=0$ == ? $d^0.5/dx^0.5 y -4sin(y)exp(x)=0$ == ? $(d^0.5y)/dx^0.5=sin(x^2)y$ == ? $(d^0.5y)/dx^0.5-sin(x)sin(y)=0$ == ? $(d^0.5y)/dx^0.5-sinh(x)sinh(y)=0$ == ? $y^((1))(x)=exp(x-y)-x$ == ? $x(d^0.5y)/dx^0.5-y-2x=0$ == ? $(d^0.5y)/dx^0.5=sinh(x-1)sinh(y-1)$ == ? $y^((0.5))(x)-exp(-x)y^2=0$ == ? $(d^0.5y)/dx^0.5=y/xsin(x)$ == ? $(dy)/dx-sin(x-y)-1=0$ == ? $(d^2.5y)/dx^2.5=y(d^0.5y)/dx^0.5$ == ? $(d^0.5y)/dx^0.5=y(dy)/dx$ == ? $(d^(2-i)y)/dx^(2-i)- y+x=0$ == ? $(d^2y)/dx^2=y^3x^2$ == ? $y(d^2y)/dx^2-x^2-3x-1=0$ == ? $y(d^2y)/dx^2-2x^2-3x-1=0$ == ? $(y-x-1)(d^2y)/dx^2-3x-1=0$ == ? $y^2(d^2y)/dx^2-x^2-4x-4=0$ == ? $(y-x-1)(d^2y)/dx^2-x^2-4x-4=0$ == ? $y(d^2y)/dx^2-2x^2-2x-1=0$ == ? $y(d^3y)/dx^3-6x^3-3x^2-3x-1=0$ == ? $y^((0))(x)y^((1))(x)y^((2))(x)=x^2$ == ? $y^((3))(x)y^((2))(x)=y^((1/2))(x)$ == ? $y^((3))(x)=exp(x)y^((1))(x)y^((1/2))(x)$ == ? $y^((1/2))(x)y^((3))(x)=exp(x)$ == ? $y^((1/2))(x)y^((2))(x)=exp(x)$ == ? $(d^0.5y)/dx^0.5-2xy-1=0$ == ? $y^2(d^0.5y)/dx^0.5-x^2-4x-4=0$ == ? $exp(y-1)(d^0.5y)/dx^0.5-x=0$ == ? $y(d^2y)/dx^2-(x-2)(2x-4)=0$ == ? $y(d^3y)/dx^3-6x^3-4x^2-4x-1=0$ == ? $exp(y-1)(d^2y)/dx^2-exp(x)=0$ == ? $y^2(d^2y)/dx^2-x^2-1=0$ == ? $1/y^2(d^2y)/dx^2-x^2-1=0$ == ? $(y-x-1)(d^3y)/dx^3-(x-2)(2x-4)(3x-1)=0$ == ? $(d^0.5y)/dx^0.5-2x^2y^2-8x^2=0$ == ? $(d^0.5y)/dx^0.5-2xy^2-8x=0$ == ? $(d^0.5y)/dx^0.5-y^2-2y-2=0$ == ? $(d^0.5y)/dx^0.5-log(y-1)exp(x)=0$ == ? $y(d^2y)/dx^2-(dy/dx)^2-1=0$ == ? $(d^2y)/dx^2-asin(y-1)-sin(x)-x=0$ == ? $dy/dx(x--y)-x--y-1 = 0$ == ?

射影定理 （又叫欧几里德(Euclid)定理）

直角三角形射影定理

直角三角形射影定理的证明

任意三角形射影定理

任意三角形射影定理与余弦定理的关系

面积射影定理

问题 ？

射影定理（又叫欧几里德(Euclid)定理）

问题？