b.( x² + 1 )² - 4x²
= ( x² + 1 )² - (2x)²
= ( x² + 1 - 2x )( x² + 1 + 2x )
= ( x² + 1 - 2x )( x² + 2.x.1 + 1² )
= ( x² + 1 - 2x )( x + 1)²
c. x² - 10x - 9y² + 25
= ( x² - 10x + 25 ) - 9y²
= ( x² - 2.x.5 + 5² ) - (3y)²
= ( x + 5 )² - (3y)²
= ( x + 5 - 3y )( x + 5 + 3y )
d. 4x² - 36x + 56
= 4x² - 8x - 28x + 56
= ( 4x² - 8x ) - ( 28x - 56 )
= 4x( x - 2 ) - 28( x - 2 )
= ( x - 2 )( 4x - 28 )
= 4( x - 2 )( x - 7 )
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