The angle between the pair of planes represented by equation2x2−2y2+4z2+6xz+2yz+3xy=0is  

[c] 2x2−2y2+4z2+(6xz+2yz+3xy)=0 or     2x2+x(6z+3y)−2y2+4z2+2yz=0 x=−(6z+3y)±36z2+9y2+36yz−8(−2y2+4z2+2yz)−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−√4x=−(6z+3y)±(2z+5y)2−−−−−−−−√4 ⇒x=−(6z+3y)±(2z+5y)4 or         2x−y+2z=0,x+2y+2z=0 ∴ Angle between planes is cos−1(49).