(a2- 1) (b2- 1) + 4ab
= a2b2 - a2 - b2 + 1 + 4ab
= a2b2 + 1 + 2ab - a2 - b2 + 2ab
= ( a2b2 + 1 + 2ab ) - ( a2 + b2 - 2ab )
= ( ab + 1)2 - ( a - b )2
= [( ab + 1 ) - ( a - b )][( ab + 1 ) + ( a - b )] [ a2 - b2 = ( a + b )( a - b )]
= [ ab + 1 - a + b ][ ab + 1 + a - b ]
