Dari deret aritmatika diketahui u3=10 dan u10=31 tentukan jumlah 30 suku pertama deret tersebut

U3 = a + 2b = 10
U10 = a + 9b = 31
________________ -
                 -7b = -21
                     b = 3

a + 2b = 10
a + 6 = 10
a = 4

Sn = n/2 {2a + (n-1)b}
S30 = 30/2 {2(4) + 29(3)}
       = 15 (8 + 87)
       = 15 (95)
       = 1.425

langsung aja y

U3 = a + 2b = 10
U10 = a + 9b = 31
_____________-
-7b = -21
b = 3

a + 2b = 10
a = 10 - 2(3)
a = 10 - 6
a = 4

S30 = 30/2 (2a + 29b)
S30 = 15 (2.4 + 29.3)
S30 = 15 (8+87)
S30 = 15 (95)
S30 = 1.425

semoga berguna +_+