1)(a/2-a/3)*1/a^2 = (1/6*а)*(1/(а^2)) = (1/6)*(1/a) = 1/(6*a)
3)a-b/a+b(a/5+b/5) = a-(b/a)+b*((a+b)/5) = a-(b/a)+((ab+b^2)/5 = (5a^2-5b+a*(ab+b^2))/(5a) = (5a^2-5b+a^2*b+a*b^2)/(5*a)
5)1:(1+1/a) = 1:((a+1)/a) = a/(a+1)
