a)tính giá trị biểu thức: $A=\frac{2.1+1}{\left(1^2+1\right)^2}+\frac{2.2+1}{\left(2^2+2\right)^2}+\frac{2.3+1}{\left(3^2+3\right)^2}+...+\frac{2.2015+1}{\left(2015^2+2015\right)^2}+\frac{2.2016+1}{\left(2016^2+2016\right)^2}$A=2.1+1(12+1)2 +2.2+1(22+2)2 +2.3+1(32+3)2 +...+2.2015+1(20152+2015)2 +2.2016+1(20162+2016)2