Geometry problem

Hello guys. Today i met a problem:

Have an array length n, (-1e6 <= a[i] <= 1e6, n<=2e5) , let find max F(l,r) = 1 * a[l] + 2*a[l+1]… + k * a[r] (k = r — l + 1). I don’t know the trick to solve problem. Can you help me <3. And thanks for your help.