Evaluate the double integral ${}_{D}f(x,y)dxdy$ for the following choices of $D$ and $f$ :

a$)D=[-1,1][0,1]$ and $f(x,y)=x^2+3y^2.$

b$)D=[0,\frac{}{2}][0,\frac{}{4}]$ and $f(x,y)=sin(x)se{c}^2(y).$

c$)D=[1,3][1,2]$ and $f(x,y)=\frac{xy}{(x^2+y^2{)}^{\frac{3}{2}}}.$