Dari deret aritmatika diketahui u7+u10+u12=40, jika nilai u7=13 maka nilai beda dan S21

Un=a+(n-1)b
U7=a+6b
a+6b=13 » a=13-6b
13+U10+U12=40
U10+U12=40-13
U10+U12=27
a+9b+a+11b=27
2a+20b=27
2(13-6b)+20b=27
26-12b+20b=27
8b=1
b=1/8
a+6b=13
a=13-6/8
a=12 ¼
S21=n/2(a+Un)
U21=12 ¼ + 20(1/8)
U21= 12 ¼ + 5/2
U21=9 ¾
S21=21/2(12 ¼+ 9 ¾)
S21=21(11)
S21=231