a4 = -21 a5 = -16 d - ?
a5 = 21 d = -5 a13 - ?
a16 = 22 d = 3 a3 - ?
a8 = -29 a18 = 41 d - ?

1) a4=a1+3da5=a1+4da1+4d=–16a1+3d=–21d=52) a5=21; d=–5a5=a1+4da1=a5–4d=21+20=41a13=a1+12d=41–60=–193) a16=22; d=3a16=a1+15da1=a16–15d=22–45=–23a3=a1+2d=–23+6=–174) a8=–29; a18=41a18=a1+17da8=a1+7d41=a1+17d–29=a1+7d70=10dd=7