simp only [coeff_eq_zero_of_natDegree_lt h, zero_mul]
case neg.a.inl
R : Type u_1
inst✝ : CommRing R
p : R[X]
r : R
hp : ¬p.natDegree = 0
i : ℕ
h : (derivative p).natDegree < i
⊢ (derivative p).coeff i * r ^ (p.natDegree - (i + 1)) =
    (derivative p).coeff i * r ^ ((derivative p).natDegree - i + (p.natDegree - ((derivative p).natDegree + 1)))
no goals