Deret aritmatika dengan u3=15 dan u7 =35.jumlah 25 suku pertama

U3 = a+2b = 15
u7 = a+6b = 35 –
—————————
-4b = -20
b = -20/-4 = 5

a+ 2b = 15
a + 2.5 = 15
a = 15-10
a = 5

S25 = 1/2.25 (2.5 + (25-1)5)
= 25/2 (10+24.5)
= 25/2 (10+120)
= 25/2 (130)
= 25.65
= 1625

maaf kalo salah..