Respuesta :

ghanami
hello : 
(-6a)-(-3a) =(-3a)-(0) = (0)-(3a) = (3a)-(6a) = -3a (arthmetic sequence, the common  -  diff is : d= -3a   the first term is : U1 = 6a
a general rule for the n th term is : Un =U1+(n-1)d
Un = 6a +(n-1)(-3a) =
Un = 6a -3an+3a
Un  = -3an +9a

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