[tex]\text{First term,}~ a = 4,\\\\\text{Common difference,}~ d = 9-4 = 5\\\\\text{Nth term} = a + (n-1) d\\\\~~~~~~~~~~~~~=4+(n-1)5\\\\~~~~~~~~~~~~~= 4+5n-5\\\\~~~~~~~~~~~~~= 5n-1\\\\\text{Hence. 80th term}~ = 5(80) -1 = 399[/tex]
