Equation of a line which is parallel to the line common to the pair of lines given by 6x2−xy−12y2−0 and 15x2+14xy−8y2−0 and at a distance 7 from it is
3x+4y=35
3x+4y=−35
6x2−xy−12y2=0
⇒ (2x - 3y)(3x + 4y) = 0 (i)
and 15x2 + 14xy - 8y2 = 0
⇒ (5x - 2y) (3x + 4y) = 0 (ii)
Equation of the line common to (i) and (ii) is
3x + 4y = 0 (iii)
Equation of any line parallel to (ii) is
3x + 4y = k
Since its distance from (iii) is 7
∣∣∣k√32+42∣∣∣=7⇒k=±(7×5)=±35