3/b²-2b+1   -   6/b²-1   +   3/1+2b+b²   =   3/(b-1)²    -   6/(b-1)(b+1)   +   3/(1+b)²   =   3(b+1)²-6(b-1)×(b+1)+3(b-1)²/(b-1)²×(b+1)²    =    3(b²+2b+1)-6(b²-1)+3(b²-2b+1)/((b-1)×(b+1))²    =    3b²+6b+3-6b²+6+3b²-6b+3/(b²-1²)   =   12/(b²-1)²   =  12/(√10²-1)² =  12/(10-1)²  =   12/9²   =  12/81   =   4/27