barisan aritmetika beserta jalan/cara

U5=7
U5=U1+(n-1)b
7= u1+(5-1)b
7= u1+4b......1

u8=13
u8=U1+(n-1)b
13= U1+(8-1)b
13= u1+7b.....2

eliminasi 1 dan 2
7= u1+4b
13= u1+7b
_________-
-6=-3b
b=2

subtitusi b=2 ke prs 1
7= u1+4b
7= u1+4x2
7-8 =u1
u1= -1

u20 = U1+(n-1)b
= -1+(20-1)2
=-1+19x2
= -1+38
=37 (B)