tolong bantu jawab yah

1. U4= 91
U6=85
U4 = a+3b
91=a+3b.... (1)
U6= a+5b
85=a+5b.... (2)

eliminasi persamaan (1) dan (2)
6 = -3b
b = -2.... (3)

subtitusi persamaan (3) ke (1)
91=a+3(-2)
a= 91+6
a=97

U20 = a+19b
=97+ 19(-2)
=97-38
=55

2. U3=58. U6=49
3b= -9
b= -3
a=58 + 9 = 67
sn=n/2(2a+(n-1)b
s20=10(134-57)
=10(74)
=740