M =( A + B) (2 ≤ A+ B) (2+ A− B)2 =2A 2B 2+ ≤
81. Ta cĩ :
M
=
(
a
+
b
) (
2
≤
a
+
b
) (
2
+
a
−
b
)
2
=
2a 2b 2
+
≤
.
=
= ⇔
⇔ = =
1
a
b
max M 2
a b
+ =
.
a b 1
2
81. Ta cĩ :
M
=
(
a
+
b
) (
2
≤
a
+
b
) (
2
+
a
−
b
)
2
=
2a 2b 2
+
≤
.
=
= ⇔
⇔ = =
1
a
b
max M 2
a b
+ =
.
a b 1
2