Algebra
posted by Faith .
Name the constant differences of the sequence 20, 21, 26, 35, 48...
What does "constant differences" mean?
Thank you so much! :)

suppose you subtract the second  first = 1
third  second = 5
fourth  third = 9
fifth  fourth = 13
to get the "first differences" of 1,5,9,13
now suppose we repeat the same procedure
51 = 4
95 = 4
139 = 4
notice we now have "constant differences" but we had to do it twice.
general concept:
if the first differences are constant, then the data came from a first degree relation
if the second differences are constant, then the data came from a second degree, or quadratic relation
if the third .... 
This site may help.
http://www.icoachmath.com/SiteMap/ConstantFunction.html