diff --git a/Mathlib/Analysis/Asymptotics/SpecificAsymptotics.lean b/Mathlib/Analysis/Asymptotics/SpecificAsymptotics.lean
index 43ba4e5458f0c8..e9d6760992c10a 100644
--- a/Mathlib/Analysis/Asymptotics/SpecificAsymptotics.lean
+++ b/Mathlib/Analysis/Asymptotics/SpecificAsymptotics.lean
@@ -44,18 +44,9 @@ open Bornology
 
 theorem Asymptotics.isLittleO_pow_pow_cobounded_of_lt (hpq : p < q) :
     (· ^ p) =o[cobounded R] (· ^ q) := by
-  nontriviality R
-  have noc : NormOneClass R := NormMulClass.toNormOneClass
-  refine IsLittleO.of_bound fun c cpos ↦ ?_
-  rw [← Nat.sub_add_cancel hpq.le]
-  simp_rw [pow_add, norm_mul, norm_pow, eventually_iff_exists_mem]
-  refine ⟨{y | c⁻¹ ≤ ‖y‖ ^ (q - p)}, ?_, fun y my ↦ ?_⟩
-  · have key : Tendsto (‖·‖ ^ (q - p)) (cobounded R) atTop :=
-      (tendsto_pow_atTop (Nat.sub_ne_zero_iff_lt.mpr hpq)).comp tendsto_norm_cobounded_atTop
-    rw [tendsto_atTop] at key
-    exact mem_map.mp (key c⁻¹)
-  · rw [← inv_mul_le_iff₀ cpos]
-    gcongr; exact my
+  rw [← Nat.add_sub_of_le hpq.le]
+  simpa [pow_add] using (isBigO_refl (· ^ p) (cobounded R)).mul_isLittleO
+    ((isLittleO_const_id_cobounded 1).pow (Nat.sub_pos_of_lt hpq))
 
 theorem Asymptotics.isBigO_pow_pow_cobounded_of_le (hpq : p ≤ q) :
     (· ^ p) =O[cobounded R] (· ^ q) := by
diff --git a/Mathlib/Analysis/CStarAlgebra/Unitization.lean b/Mathlib/Analysis/CStarAlgebra/Unitization.lean
index 6e789d584bd5a7..0830c1c2a863ea 100644
--- a/Mathlib/Analysis/CStarAlgebra/Unitization.lean
+++ b/Mathlib/Analysis/CStarAlgebra/Unitization.lean
@@ -169,6 +169,7 @@ instance Unitization.instCStarRing : CStarRing (Unitization 𝕜 E) where
     rw [h₂, h₃]
     /- use the definition of the norm, and split into cases based on whether the norm in the first
     coordinate is bigger or smaller than the norm in the second coordinate. -/
+    -- `grind` works here but is much slower
     by_cases! h : ‖(Unitization.splitMul 𝕜 E x).fst‖ ≤ ‖(Unitization.splitMul 𝕜 E x).snd‖
     · rw [sq, sq, sup_eq_right.mpr h, sup_eq_right.mpr (mul_self_le_mul_self (norm_nonneg _) h)]
     · replace h := h.le
diff --git a/Mathlib/Analysis/Complex/Trigonometric.lean b/Mathlib/Analysis/Complex/Trigonometric.lean
index 9d95e54fc24346..aed73f44f005ac 100644
--- a/Mathlib/Analysis/Complex/Trigonometric.lean
+++ b/Mathlib/Analysis/Complex/Trigonometric.lean
@@ -501,20 +501,11 @@ theorem tan_sq_div_one_add_tan_sq {x : ℂ} (hx : cos x ≠ 0) :
   simp only [← tan_mul_cos hx, mul_pow, ← inv_one_add_tan_sq hx, div_eq_mul_inv]
 
 theorem cos_three_mul : cos (3 * x) = 4 * cos x ^ 3 - 3 * cos x := by
-  have h1 : x + 2 * x = 3 * x := by ring
-  rw [← h1, cos_add x (2 * x)]
-  simp only [cos_two_mul, sin_two_mul, mul_sub, mul_one, sq]
-  have h2 : 4 * cos x ^ 3 = 2 * cos x * cos x * cos x + 2 * cos x * cos x ^ 2 := by ring
-  rw [h2, cos_sq']
-  ring
+  rw [← cosh_mul_I, mul_assoc, cosh_three_mul, cosh_mul_I]
 
 theorem sin_three_mul : sin (3 * x) = 3 * sin x - 4 * sin x ^ 3 := by
-  have h1 : x + 2 * x = 3 * x := by ring
-  rw [← h1, sin_add x (2 * x)]
-  simp only [cos_two_mul, sin_two_mul, cos_sq']
-  have h2 : cos x * (2 * sin x * cos x) = 2 * sin x * cos x ^ 2 := by ring
-  rw [h2, cos_sq']
-  ring
+  rw [← two_add_one_eq_three, add_one_mul, sin_add (2 * x) x]
+  grind [cos_two_mul, sin_two_mul, cos_sq']
 
 theorem exp_mul_I : exp (x * I) = cos x + sin x * I :=
   (cos_add_sin_I _).symm