(49a² -1)*(1/(7a+1) -1/(7a - 1)) =(49a² -1)*(7a -1 -(7a+1)/(7a+1)(7a - 1) =
(49a² -1)*(-2)/(49a² -1) = - 2. не зависит от значения переменного а .
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или (49a² -1)*(1/(7a+1) -1/(7a - 1)) =(7a+1)*(7a -1)(1/(7a+1) -1/(7a - 1)) =
(7a+1)*(7a -1)(1/(7a+1) - (7a+1)*(7a-1)* 1/(7a - 1) =
7a -1 - (7a+1) = -2 .