tentukan suku pertama dan beda a.) U10 = -4 ; U5 = 21 b.) U4 = -9 ; U15 = -31

a)
U10 = a + 9b = -4
U5 = a + 4b = 21
—————————— -
5b = -25
b = 5
U5 = a + 4(5) = 21
a. = 21 - 20
a = 1

b)
U4 = a + 3b = -9
U15 = a + 14b = -31
—————————— -
-11b = 22
b = -2
U4 = a + 3(-2) = -9
a = -9 -6
a = -15