The condition that the normals at the points whose eccentric angles are α, β, γ shall be concurrent is

The following method of establishing this result, as compared with those given in works on Concis, is direct, and also has the advantage of simplicity, by first proving the Trigonometrical identity

which may be simply done by multiplying both sides of the wellknown identity

when it will be easily found that the product in the first member reduces to that in (A) with opposite sign.