6. Prove: If f
ˉ
​
∈R(α) on [a,b] and if a ˉ
​
∈R(α) on [a,c] and on [c,b] and ∫ a
b
​
f
ˉ
​
dα=∫ a
c
​
f
ˉ
​
dα+∫ c
b
​
f
ˉ
​
dα.(3 points).