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A history of Greek mathematics - Wilbourhall.org

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.<br />

THE GONIG& BOOK VII 173<br />

conjugate diameters in an ellipse or conjugate hyperbolas, and<br />

if the tangents at their extremities form the parallelogram<br />

LL'MM', then<br />

the parallelogram LL'MM' — rect. AA' .<br />

BB'<br />

The pro<strong>of</strong> is interesting. Let the tangents at P, D respectively<br />

meet the major or transverse axis in T, T'.<br />

Now (by VII. 4) PT 2 : GD 2 = NT:GN\<br />

therefore 2 A GPT : 2 A T'DC = NT :<br />

CN.<br />

But<br />

2AGPT:(GL) = PT:CD,<br />

= OP :<br />

DT'<br />

, by similar triangles,<br />

= (CL):2AT'DG.<br />

That is, (OX) is a mean proportional between 2AGPT and<br />

2ArDC.<br />

Therefore, since >/(NT . OiV) is a mean proportional between<br />

iVT and CUV,

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