Nilai a dan b dengan rumus un=a+(n-1)b, jika u4=20, u3=9, u15=75, u6-u1=10

U4 = a + 3b = 20
U3 = a + 2b = 9
-------------------------(-)
b = 11
a + 2b = 9
a + 2(11) = 9
a + 22 = 9
a = 9 - 22
a = - 13

maka
a = - 13
b = 11

U3 = a + 2b = 9
U4 = a + 3b = 20
-
-b = -11
b = 11

U3 = a + 2b
9 = a + 2 (11)
9 = a + 22
a = 22 - 9
a = 13

Semoga membantu.. :)