Diketahui u6+u10=62 U12+uu8=12 Ditanyakan s3?

un=a+(n-1)b
u6+u10=62
a+5b+a+9b=62
2a+14b=62

u12-u8=12
a+11b-(a+7b)=12
a+11b-a-7b=12
4b=12
b=3

2a+14b=62
2a+14(3)=62
2a=62-42
2a=20
a=10

sn=1/2n(2a+(n-1)b)
s3=3/2(2(10)+(3-1)3)
=3/2(20+6)
=3/2(26)
=3×13=39