diff --git a/src/Init/Data/Array/Find.lean b/src/Init/Data/Array/Find.lean
index 881329c970..d8e7091852 100644
--- a/src/Init/Data/Array/Find.lean
+++ b/src/Init/Data/Array/Find.lean
@@ -743,13 +743,15 @@ theorem finIdxOf?_empty [BEq α] : (#[] : Array α).finIdxOf? a = none := by sim
   simp [List.finIdxOf?_eq_some_iff]
 
 @[simp]
-theorem isSome_finIdxOf? [BEq α] [LawfulBEq α] {xs : Array α} {a : α} :
-    (xs.finIdxOf? a).isSome ↔ a ∈ xs := by
+theorem isSome_finIdxOf? [BEq α] [PartialEquivBEq α] {xs : Array α} {a : α} :
+    (xs.finIdxOf? a).isSome = xs.contains a := by
   rcases xs with ⟨xs⟩
   simp [Array.size]
 
-theorem isNone_finIdxOf? [BEq α] [LawfulBEq α] {xs : Array α} {a : α} :
-    (xs.finIdxOf? a).isNone = ¬ a ∈ xs := by
-  simp
+@[simp]
+theorem isNone_finIdxOf? [BEq α] [PartialEquivBEq α] {xs : Array α} {a : α} :
+    (xs.finIdxOf? a).isNone = !xs.contains a := by
+  rcases xs with ⟨xs⟩
+  simp [Array.size]
 
 end Array
diff --git a/src/Init/Data/List/Find.lean b/src/Init/Data/List/Find.lean
index 56cdf2ad7f..227dfcbeee 100644
--- a/src/Init/Data/List/Find.lean
+++ b/src/Init/Data/List/Find.lean
@@ -1097,17 +1097,18 @@ theorem finIdxOf?_cons [BEq α] {a : α} {xs : List α} :
   simp only [finIdxOf?, findFinIdx?_eq_some_iff, beq_iff_eq]
 
 @[simp]
-theorem isSome_finIdxOf? [BEq α] [LawfulBEq α] {l : List α} {a : α} :
-    (l.finIdxOf? a).isSome ↔ a ∈ l := by
+theorem isSome_finIdxOf? [BEq α] [PartialEquivBEq α] {l : List α} {a : α} :
+    (l.finIdxOf? a).isSome = l.contains a := by
   induction l with
   | nil => simp
   | cons x xs ih =>
     simp only [finIdxOf?_cons]
-    split <;> simp_all [@eq_comm _ x a]
+    split <;> simp_all [BEq.comm]
 
-theorem isNone_finIdxOf? [BEq α] [LawfulBEq α] {l : List α} {a : α} :
-    (l.finIdxOf? a).isNone = ¬ a ∈ l := by
-  simp
+@[simp]
+theorem isNone_finIdxOf? [BEq α] [PartialEquivBEq α] {l : List α} {a : α} :
+    (l.finIdxOf? a).isNone = !l.contains a := by
+  rw [← isSome_finIdxOf?, Option.not_isSome]
 
 /-! ### idxOf?