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task/30121970

12. (a - x) /x =1 ⇔ a/x =2.    (a+x)/a = 1 +x/a =1 +1/2 = 1,5  .

12. (x+y) /y =4 ⇔x+y =4y ⇒x  =3y.   y / (x - y) = y / (3y - y) =y/2y = 0.5  .

9. ( 7/( b+7) +(b²+49) / (b²- 49) +7 / (7 -b) ) : (b+1) / 2 =

[ ( 7(b-7) +(b²+49) - 7(b+7) ) / (b²- 49) ] : (b+1) / 2 =

[ ( 7b - 49 + b²+ 49 - 7b - 49 ) / (b²- 49) ] : (b+1) / 2 =

[ ( b² - 49) / (b²- 49) ]  *2 / (b+1) ] =2 / (b+1) .