dalam sebuah deret aritmetika diketahui a=3,Un=73 dan u8+u11=91.tentukanlah n,sn dan s12

Diketahui :
a = 3
Un = 73
U8 + U11 = 91

Mencari beda 
U8 = a + 7b 
U11 = a + 10 b

U8 + U11 >> a + 7b + a + 10b = 91
a  + 7b + a + 10b = 91
3 + 7b + 3 + 10b = 91
6 + 17b = 91
17b = 91 - 6
17b = 85
b = 5

Mencari n
Un = a + (n-1) b
73 = 3 + (n-1) 5
73 = 3 + 5n - 5
73 = -2 + 5n
73 + 2 = 5n
75 = 5n
n = 75/5
n = 15

Rumus Sn
Sn = n/2 (2a + (n-1) b)
S15 = 15/2 (2(3) + (15-1) 5)
S15 = 15/2 (6 + (14) 5)
S15 = 15/2 (6 + 70)
S15 = 15/2 (76)
S15 = 570

Mencari S12
Sn = n/2 (2a + (n-1) b)
S12 = 12 / 2 (2(3) +(12 - 1) 5)
S12 = 6 (6 + (11) 5)
S12= 6 (6 + 55)
S12 = 6 (61)
S12 = 366

a=3
(a+7b)+(a+10b)=91
2a+17b=91
2(3)+17b=91
17b=85
b=5

Un=73
a+(n-1)b=73
3+(n-1)5=73
5n-2=73
5n=75
n=15

Sn=(n/2)(2a+(n-1)b)
Sn=(n/2)(6+5n-5)
Sn=(n/2)(5n+1)=(5n²+n)/2

S12=(5(12)²+12)/2=(5(144)+12)/2=(720+12)/2=132/2=66