Tolong bantu jawab yaa

1. U21 = a+(n-1)b
1+(21-1)2
1+(20)2
1+40
=41
2. (Caranya di coba coba hehe, ini pake pilihan yg D)
=n/2(2a+(n-1)b
=9/2(2.3+(9-1)4
=4,5(6+(8)4
=4,5(6+32)
=4,5(38)
=171