have hmn : n ≤ n + m := Nat.le_add_right n m
n m : ℕ
f : ℕ → ℚ
⊢ ∑ i ∈ Finset.Ico n (n + 2 * m), f i = ∑ i ∈ Finset.Ico n (n + m), f i + ∑ i ∈ Finset.Ico (n + m) (n + 2 * m), f i
n m : ℕ
f : ℕ → ℚ
hmn : n ≤ n + m
⊢ ∑ i ∈ Finset.Ico n (n + 2 * m), f i = ∑ i ∈ Finset.Ico n (n + m), f i + ∑ i ∈ Finset.Ico (n + m) (n + 2 * m), f i