What we suppose works for all Riemann-integrable function , and we only know that .
f is continuous and g is integrable and the product of the integral of fg over the interval [a,b] is 0. So f(x) must be 0 for all x in [a,b], is all the information I have been given. I will start with the definition of continuous for f and integrable for g and then hope I see something useful after that.
Suppose that is not for all , then there exists such that (If the is no such , there must be such that .In that case we will look at ).
f is continuous function therefor exists for which for all .
Now we define -'staircase'-characteristic function of open interval .
Hence, we will get:
So, we got a contradiction to the fact that if is continuous on and h is staircase function .
But we also know that if is integrable function on , so for every exists staircase function so that
Combining these two results we will conclude the needed.