3x-2y=27-xy

3x-2y+xy=27

3x+xy-2y=27

x(3+y)-2y-6=27-6

x(3+y)-(2y+6)=21

x(3+y)-2(y+3)=21

(y+3)(x-2)=21

Ta có bảng giá trị:

Ta thấy: B=\(\frac{2015+2016}{2016+2017}\)=\(\frac{2015}{2016+2017}\)+\(\frac{2016}{2016+2017}\)

              A=\(\frac{2015}{2016}\)+\(\frac{2016}{2017}\)

Mà\(\frac{2015}{2016+2017}\)<\(\frac{2015}{2016}\); \(\frac{2016}{2016+2017}\)<\(\frac{2016}{2017}\)

Suy ra: \(\frac{2015}{2016}\)+\(\frac{2016}{2017}\)>\(\frac{2015}{2016+2017}\)+\(\frac{2016}{2016+2017}\)=\(\frac{2015+2016}{2016+2017}\)

               Hay A>B