Jika u5=15, u11=31. carilah b (dua cara) ,U55 dan s55

B = (31-15)/(11-5) = 16/6
u5 = a + 4×16/6 = 15
a + 64/6 = 90/6
a = (90-64)/6 = 26/6 = 13/3 = 4⅓

U55 = 26/6 + 54×16/6 = 4⅓ + 144 = 148⅓

S55 = 55 x ( 4⅓ + 148⅓)/2
= 55 x 152⅔/2
= 55 x 458/6
= (330+458)/6
= 788/6
= 131 ⅓