số khó 8!

Y

young_wolf

$(8x^2 + 22x +15)(4x^2 +15x +14)-3$

$=(8x^2+10x+12x+15)(4x^2+7x+8x+14)-3$

$=(2x+3)(4x+5)(4x+7)(x+2)-3$

$=[(2x+3)(4x+7)][(4x+5)(x+2)]-3$

$=(8x^2+26x+21)(4x^2+13x+10)-3$

Đặt $4x^2+13x+10=a$. Ta được:

$(2a+1)a-3=2a^2+a-3=2a^2+3a-2a-3=(a-1)(2a+3)=(4x^2+13x+9)
(8x^2+26x+23)$
 
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