Evaluate :
(i) (6−xy)(6+xy)
(ii) (7x+23y)(7x−23y)
(iii) (a2b+2ba)(a2b−2ba)
(iv) (3x−12y)(3x+12y)
(v) (2a+3)(2a−3)(4a2+9)
(vi) (a+bc)(a−bc)(a2+b2c2)
(vii) (5x+8y)(3x+5y)
(viii) (7x+15y)(5x−4y)
(ix) (2a−3b)(3a+4b)
(x) (9a−7b)(3a−b)