Find the co-ordinates of the orthocentre of the triangle, the equations of whose sides are x + y = 1,2x + 3y = 6, 4x − y + 4 = 0, without finding the co−ordinates of its vertices.
Find the equation of the pair of straight lines joining the origin to the points of intersection of the circles represented by x^2 + y^2 = a^2 and x^2 + y^2 + 2 ( gx + fy ) = 0.