2./ Ta thấy, với ∀ ∈ x ( ) a b , , ∃ ∈ t ( a x k , ) , ∈ ( ) x b , :
= − = − ≤ −
f x f x f a f t x a M x a
| ( ) | | ( ) ( ) | | '( ) | ( ) ( ).
| ( ) | | ( ) ( ) | | '( ) | ( ) ( ).
f x f b f x f k b x M b x
∫ ∫
f x dx ≤ f x dx
Suy ra b ( ) b ( ) .
a a
+ +
a b a b
b b
2 2
( ) ( ) ( ) ( )
∫ ∫ ∫ ∫
= + ≤ − + −
f x dx f x dx M x a dx b x dx
+ −
a b b
2
a b
M x a b x a b M
( ) ( ) ( )
= − − − + =
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