下列运算，结果正确的是
$a^2+a^2=a^4$                           ${\left(a-b\right)}^2=a^2-b^2$

$2\left(a+b\right)+\left(ab\right)=2a$                     ${\left(3a{b}^2\right)}^2=6a^2{b}^4$