Come si scrive una regola per l'ennesimo termine della sequenza aritmetica dato d = -3, # a_2 = 18 #?

#"the nth term of an arithmetic sequence is"#

#•color(white)(x)a_n=a+(n-1)d#

#"where a is the first term and d the common difference"#

#"we have to find a"#

#a_2=a+d=18#

#rArra-3=18rArra=21#

#rArra_n=21-3(n-1)#

#color(white)(rArra_n)=21-3n+3#

#rArra_n=24-3n#