A) b=4, U6=21 , a=... B) a=-5, U20=33, b=... C) a=9, b=-2, un=-19, n=... D) U4=1, U7=-8, a=...... ,b=..... #caranya kaaa

A) U6 = 21 → a+5b

beda = 4

suku pertama (a) :
a+5b=21
a + 20 = 21
a = 1

B) U20 = 33 → a+19b

suku pertama (a) = -5

beda :
a+19b=33
-5+19b=33
19b=38
b = 2

C) suku pertama (a) = 9
beda = -2
Un = -19

nilai n :
Un=a+(n-1)b
-19 = 9 + (n-1) -2
-19 = 9 -2n + 2
-19 = -2n + 11
2n = 30
n = 15

D) U4 =1 → a+3b
U7 = -8 → a+6b

beda :
a+6b=-8
a+3b=1
————— —
3b = -9
b = -3

suku pertama (a) :
a+3b=1
a+(-9)=1
a=10