1) [ 1 / ( 3B - 7) ] - [ 1 / ( 3B + 7 ) ] = ( 3B + 7 - ( 3B - 7) ) / ( 9B^2 - 49 ) =
= 14 / ( 9B^2 - 49 )
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2) (9B^2 - 49) * [ 14 / ( 9B^2 - 49 ) ] = 14
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3) 14 + B - 13 = 1 + B
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4) B = 345
1 + 345 = 346
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Ответ 346