9a² + b² - 6ab = ( 3a - b )²
b² - 9a² = ( b - 3a )( b + 3a )
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( 3a - b )² / ( ( b - 3a )( b + 3a )) = ( 3a - b ) / ( - ( b + 3a )) = ( 3a - b )/( - 3a - b )
a = 1/7 ; b = 4/7
3a - b = 3*( 1/7 ) - 4/7 = 3/7 - 4/7 = - 1/7
- 3a - b = - 3*( 1/7 ) - 4/7 = - 3/7 - 4/7 = - 7/7 = - 1
- 1/7 : ( - 1 ) = 1/7
Ответ 1/7