U5=10, U7=0, dicari suku pertama dan beda

U5 = a + 4b = 10
U7 = a + 6b = 0
_________ –
-2b = 10
b = 10/(-2) = -5

a + 4b = 10
a + (4×(-5)) = 10
a – 20 = 10
a = 10 + 20
a = 30

U1 = a = 30
beda = -20