toán 9 - ôn tập

H

huuthuyenrop2

ta có:
$a^4-b^4=a^4-1-(b^4-1)$
$=(a^2-1)(a^2+1)-(b^2-1)(b^2+1)$
$=(a-1)(a+1)(a^2-4+5)-(b-1)(b+1)(b^2-4+5)$
$=(a-1)(a+1)(a-2)(a+2)+5(a-1)(a+1)-(b-1)(b+1)(b-2)(b+2)+5(b-1)(b+1)$
vì a,b ko chia hêt cho 5
$\Rightarrow dpcm$
 
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