Tính các giới hạn sau

Tính các giới hạn sau :
a) lim $\left(1-\frac{4}{1^2}\right)\left(1-\frac{4}{3^2}\right)...\left(1-\frac{4}{{\left(2n-1\right)}^2}\right)$​
b) lim $\frac{2.1^2+3.2^2+...+\left(n+1\right)n^2}{n^4}$
c) lim $\frac{1.3.4+2.5.7+3.7.10+...+n\left(2n+1\right)\left(3n+2\right)}{n^4}$
d) lim $\left[\left(\frac{1^2}{1.3}+\frac{2^2}{3.5}+\frac{3^2}{5.7}+...+\frac{n^2}{\left(2n-1\right)\left(2n+1\right)}\right)\frac{1}{2}\right]$