[25/(a²+5a+25) - 2a/(5-a) - (a³+25a²)/(a³-125)]*[a-5 + 15a/(a-5)]=a-5
1)25/(a²+5a+25) + 2a/(a-5)) - (a³+25a²)/(a³-125)=
=(25(a-5)+2a(a²+5a+25)-(a³+25a²))/(a³-125)=
=(25a-125+2a³+10a²+50a-a³-25a²)/(a³-125)=(a³-15a²+75a-125)/(a³-125)=
=[(a³-125)-(15a²-75a)]/(a³-125)=[(a-5)(a²+5a+25)-15a(a-5)]/(a³-125)=
=(a-5)(a²+5a+25-15a)/(a-5)(a²+5a+125)=(a²-10a+25)/(a²+5a+25)=(a-5)²/(a²+5a+25)
2)a-5 + 15a/(a-5)=(a²-10a+25+15a)/(a-5)=(a²+5a+25)/(a-5)
3)(a-5)²/(a²+5a+25) *(a²+5a+25)/(a-5)=(a-5)