Ada yang ngerti gk kak? kasih tau caranya ya

U3 = 9
U5 + U7 = U12 = 36
Eliminasi :
a + 2b = 9
a + 11b = 36 -
= -9b = -27
= b =-27/-9
= b = 3
Substitusi :
a + 2b = 9
= a + 2(3) = 9
= a + 6 = 9
= a = 9 - 6 = 3
Rumus : Sn = n/2 (2a + (n-1) b)
S10 = 10/2 (2(3) + (10-1) 3)
= 5 ( 6+ (9×3) )
= 5 ( 6 + 27 )
= 5 × 33
= 165