[tex]a=5\\\\a)\\\\ 2a^2=2*5^2=2*25=50\\\\b) \\\\5a^2-4a+120=5*5^2-4*5+120=5*25-20+120=125+100=225\\\\c)\\\\\frac{ 3a^2+8}{2}= \frac{ 3*5^2+8}{2}= \frac{ 3*25+8}{2}=\frac{ 75+8}{2}=\frac{83}{2}=41\frac{1}{2}\\\\d)\\\\\frac{6a^2-a}{12}=\frac{6*5^2-5}{12}=\frac{6*25-5}{12}=\frac{150-5}{12}=\frac{145}{12}=12\frac{1}{12}[/tex]
