\(a.\\ L=\dfrac{N}{2}.3,4=\dfrac{1200}{2}.3,4=2040\left(A^o\right)\\ C=\dfrac{N}{20}=\dfrac{1200}{20}=60\left(chu.kì\right)b.\\ N=2A+2G=1200\left(Nu\right)\\ H=2A+3G=1680\left(liên.kết\right)\\ \Rightarrow G=X=480\left(Nu\right);A=T=120\left(Nu\right)\\ \)