(4a-15)/(3a) - (a-5)(a+5)/(a*(a-5))=
=(4a-15)/(3a) - (a+5)/(a)=
=(4a-15)/(3a) -(3*(a+5))/(3a)=
=(4a-15-3a-15)/(3a) - (a+5)/(a)=(a-30)/(3a)
1/3 получится, если в знаменателе второй дроби а+5
(4a-15)/(3a) - (a-5)(a+5)/(a*(a+5))=
=(4a-15)/(3a) - (a-5)/(a)=
=(4a-15)/(3a) -(3*(a-5))/(3a)=
=(4a-15-3a+15)/(3a) - (a+5)/(a)=(a)/(3a)=1/3
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