Pair of Tangents from an External Point

Pair of Tangents from an External Point

For:

$$y^2=4ax$$

the combined equation of the two tangents from $$(x_1,y_1)$$ is:

$$T^2=SS_1$$

where:

$$S=y^2-4ax$$

and:

$$T=yy_1-2a(x+x_1)$$

Usage

- Used when two tangents are drawn from an external point and their combined equation is required.