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Mar 10, 2016
 #1
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Mar 10, 2016
 #3
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continuation

\(\boxed{~ \begin{array}{rcll} x_{1,2} &=& \dfrac{ -[~ 1+b\cdot f'(x_a) ~] \pm \sqrt{ 1+ [~ f'(x_a) ~]^2 \cdot [~ 2+ b^2 -4\cdot a\cdot(c-y_a) ~] } } { 2\cdot a \cdot f'(x_a) } \\\\ && y_a = a\cdot x_a^2+b\cdot x_a+c \\\\ && f'(x_a) = 2a\cdot x_a +b \\\\ x_1 + x_2 &=& \dfrac{ -[~ 1+b\cdot f'(x_a) ~] + \sqrt{ 1+ [~ f'(x_a) ~]^2 \cdot [~ 2+ b^2 -4\cdot a\cdot(c-y_a) ~] } } { 2\cdot a \cdot f'(x_a) } \\ && +\dfrac{ -[~ 1+b\cdot f'(x_a) ~] - \sqrt{ 1+ [~ f'(x_a) ~]^2 \cdot [~ 2+ b^2 -4\cdot a\cdot(c-y_a) ~] } } { 2\cdot a \cdot f'(x_a) } \\\\ x_1 + x_2 &=& \dfrac{ -[~ 1+b\cdot f'(x_a) ~] } { 2\cdot a \cdot f'(x_a) } + \dfrac{ -[~ 1+b\cdot f'(x_a) ~] } { 2\cdot a \cdot f'(x_a) } \\\\ x_1 + x_2 &=& \dfrac{ -2\cdot [~ 1+b\cdot f'(x_a) ~] } { 2\cdot a \cdot f'(x_a) } \\\\ x_1 + x_2 &=& -\dfrac{ [~ 1+b\cdot f'(x_a) ~] } { a \cdot f'(x_a) } \\\\ x_1 + x_2 &=& -\frac{1}{a} \cdot \left[ \dfrac{ 1 } { f'(x_a) } + b \right] \qquad | \qquad f'(x_a) = 2ax_a+b \\\\ x_1 + x_2 &=& -\frac{1}{a} \cdot \left( \dfrac{ 1 } { 2ax_a+b } + b \right) \qquad | \qquad x_1 = x_a \qquad x_2 = x_b \\\\ x_a + x_b &=& -\frac{1}{a} \cdot \left( \dfrac{ 1 } { 2ax_a+b } + b \right) \\\\ x_b &=& -x_a -\frac{1}{a} \cdot \left( \dfrac{ 1 } { 2ax_a+b } + b \right) \\\\ \end{array} ~}\)

 

conclusion

\(\boxed{~ \begin{array}{lcll} \text{Parabola } \quad y = ax^2+bx+c \qquad \text{ and we have } \quad a,b,c \text{ and } x_a \\\\ \Rightarrow x_b = -x_a -\frac{1}{a} \cdot \left( \dfrac{ 1 } { 2ax_a+b } + b \right) \\\\ \Rightarrow y_b = ax_b^2+bx_b+c \\\\ \end{array} ~}\)

 

Example:

\(\begin{array}{rcll} \text{Parabola } \quad y &=& \frac12 x^2 \qquad a=\frac12 \qquad b=0 \qquad c=0 \qquad x_a=4 \\\\ x_b &=& -4 -\frac{1}{\frac12} \cdot \left( \dfrac{ 1 } { 2\cdot \frac12 \cdot 4+0 } + 0 \right) \\\\ x_b &=& -4 -\frac{1}{\frac12} \cdot \left( \dfrac{ 1 } { 4 } \right) \\\\ x_b &=& -4 - \frac24 \\\\ x_b &=& -4 - \frac12 \\\\ \mathbf{x_b} & \mathbf{=} & \mathbf{-4.5} \\\\\\ y_b &=& \frac12 \cdot (-4.5)^2+0\cdot (-4.5)+0 \\\\ \mathbf{y_b} &\mathbf{=}& \mathbf{10.125} \\\\ \end{array}\)

 

laugh

Mar 10, 2016
 #4
avatar+333 
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Mar 10, 2016

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