`A + B = C`

`A = (x_a, y_a)`

`B = (x_b, y_b)`

`C = (x_c, y_c)`

`λ = (y_b - y_a)/(x_b - x_a) = (y_b - y_a)(x_b - x_a)^{-1} = (y_b - y_a)(x_b - x_a)^{p-2} (mod p)`

`x_c = λ^2 + a_1λ - a_2 - x_a - x_b`

`y_c = -a_1x_c - a_3 - λx_c + λx_a - y_a`

$$Y^2 + a_1XY + a_3Y = X^3 + a_2X^2 + a_4X + a_6$$