a,
$\lim\Big( 0,5^n+\dfrac{3}{n^2}\Big)=0+0=0$
b,
$\lim\dfrac{ 0,75^n}{1+\dfrac{1}{n}}=\dfrac{0}{1+0}=0$
c,
$\lim\dfrac{1}{n^2}.\dfrac{1}{2^n}=0.0=0$
d,
$\lim\dfrac{n^2-6n+1}{2n^2+n+3}=\lim\dfrac{ 1-\dfrac{6}{n}+\dfrac{1}{n^2}}{2+\dfrac{1}{n}+\dfrac{3}{n^2}}=\dfrac{1}{2}$