U5=40 U8=13 hitung jumlan 12suku pertama

U5=a+4b=40
U8=a+7b=13
-------------------- (-)
-3b=27
b=27/(-3)
b=(-9)
b=(-9)=>a+4b=40
a+4(-9)=40
a-36=40
a=40+36
a=76
Sn=n/2(2a+(n-1)b)
S12=12/2[(2(76)+(12-1)(-9)]
=6{152+[11 x (-9)]}
=6[152+(-99)]
=6(53)
=318
maaf klo salah