Baker (1684): using a parabola to solve a biquadratic equation

This figure appears in The Geometrical key, or the gate of equations unlock'd by Thomas Baker (c.1625-c.1689), Fig. 8 (with explanation pp. 15-17). Baker extended the work of Descartes to solve polynomial equations up to the fourth degree (quadratic, cubic and quartic or biquadratic). This applet illustrates the solution of x4 + qx² = s, where q and s are given constants. The solution found by Baker’s method is shown in red, while the coefficients can be adjusted using the sliders. Baker’s work is in the collection of Marsh’s Library, Dublin.