Respuesta :

add 18 each time
first term is -108

an=a1+d(n-1)
d=common difference
a1=startng term

an=-108+18(n-1)

31 term
a31=-108+18(31-1)
a31=-108+18(30)
a31=-108+540
a31=432


31st term is 432
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