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Pair of Tangents from a Point
For:
$$y^2=4ax$$
and point $$(x_1,y_1)$$, define:
$$S=y^2-4ax$$
$$S_1=y_1^2-4ax_1$$
$$T=yy_1-2a(x+x_1)$$
Then the pair of tangents is:
$$T^2=SS_1$$
Usage
- Used when the combined equation of the two tangents from a point is required.
