MHB Prove the Given Statement....3

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The discussion revolves around proving the equation a•b + c•d = -2C for a circle defined by the equation x^2 + 2Ax + y^2 + 2By = C, where a and b are x-intercepts and c and d are y-intercepts. After solving the previous problems in the series, the user applies the product of the roots formula and manipulates the equations to derive the desired result. The proof involves substituting values and simplifying to confirm that 2ab = -2C, leading to the conclusion. The participants express gratitude for the assistance provided in reaching the solution. The discussion effectively demonstrates the application of algebraic techniques in geometry.
mathdad
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Suppose that the circle x^2 + 2Ax + y^2 + 2By = C has two intercepts, a and b, and two y-intercepts, c and d.
Prove that a•b + c•d = -2C

How is this started?
 
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Once you work the first two problems in this series, this one should be a walk in the park. :D
 
I am stuck with the second problem in the series.
 
RTCNTC said:
Suppose that the circle x^2 + 2Ax + y^2 + 2By = C has two intercepts, a and b, and two y-intercepts, c and d.
Prove that a•b + c•d = -2C

$$A=-\frac{a+b}{2}$$

and the point (a, 0) is on the circle. Plug and chug:

$$-ab=C$$

Get sneaky:

$$2ab=-2C$$

...and using the result from part 2:

$$2ab=ab+cd=-2C$$

QED
 
Nicely done.
 
If you use the product of the roots formula, you get:

$$ab+cd=-C-C=-2C$$ :D
 
Thank you, Mark. Thank you everyone.
 

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