dari suatu deret aritmetika diketahui u3+u6+u12=72 dan u1+u2=180 berapa s14

aritmetika
u3 + u6 + u12 = 72
a+ 2b+ a+ 5b+ a+ 11b = 72
3a + 18b = 72
a + 6b = 24


u1 + u2 = 180
a + a+ b = 180
2a + b = 180

a + 6b = 24 ...|kali 2
2a + b = 180

2a + 12b = 48
2a +  b = 180
........................(-)
11b = - 132
b = -12

a + 6b= 24
a = 24 - 6b
a = 24 - 6(-12)
a = 24 + 72
a = 96

U14 = a + 13b
u14 = 96 + 13(-12)
u14 = 96 - 156
u14 = -60


Sn = n/2 (a + un)
S14 = 14/2 (96 - 60)
S14 = 7(36) 
S14 = 252