請幫忙, 不明白的arithmetic mean (5分)

Insert two arithmetic means between a and b
Solution:
We have
First term a = a
Last term l = b
number of terms n = 2 + 2 = 4.
Then b = a + 3 ∙ d (where d is the difference)
d = (b-a)/3.
The first mean is a+ (b-a)/3 = (2a+b)/3
The second mean is (2a+b)/3 + (b-a)/3 = (a+2b)/3
The resulting progression is a, (2a+b)/3 , (a+2b)/3, b

2007-04-19 16:13:19 補充：
題目即是想加兩個數x,y入a,b到令a,x,y,b 成等差數列