Đáp án:
$1)a) -10x^5-15x^4+25x^3\\
b) 125x^3-75x^2y-8y^3\\
2)
a)22\\
b)
-39\\
3)
a) x=\dfrac{36}{19}\\
b)
x=1$
Giải thích các bước giải:
$1)a) -5x^3.(2x^2+3x-5)=-10x^5-15x^4+25x^3\\
b) (25x^2+10xy+4y^2).(5x-2y)\\
=(25x^2+10xy+4y^2).5x-(25x^2+10xy+4y^2).2y\\
=25x^2.5x+10xy.5x+4y^2.5x-25x^2.2y-10xy.2y-4y^2.2y\\
=125x^3+50x^2y+20xy^2-125x^2y-20xy^2-8y^3\\
=125x^3+(50x^2y-125x^2y)+(20xy^2-20xy^2)-8y^3\\
=125x^3-75x^2y-8y^3\\
2)
a) x^2(x+y)-y(x^2-y)+20\\
=x^3+x^2y-x^2y+y^2+20\\
=x^3+y^2+20\\
\Rightarrow 1^3+(-1)^2+20=22\\
b)
(x-3)(x+7)+(5-2x)(x-1)\\
=x^2+7x-3x-21+5x-5-2x^2+2x\\
=(x^2-2x^2)+(7x-3x+5x+2x)+(-21-5)\\
=-x^2+11x-26\\
\Rightarrow -(-1)^2+11.(-1)-26=-1-11-26=-39\\
3)
a) 3(2x-1)-5(x-3)+6(3x-4)=24\\
\Leftrightarrow 6x-3-(5x-15)+18x-24=24\\
\Leftrightarrow 6x-3-5x+15+18x-24=24\\
\Leftrightarrow (6x-5x+18x)+(-3+15-24)=24\\
\Leftrightarrow 19x-12=24\\
\Leftrightarrow 19x=24+12=36\\
\Leftrightarrow x=\dfrac{36}{19}\\
b)
(3x-1)(2x+7)-(x+1)(6x-5)=16\\
\Leftrightarrow 6x^2+21x-2x-7-(6x^2-5x+6x-5)=16\\
\Leftrightarrow 6x^2+21x-2x-7-6x^2+5x-6x+5=16\\
\Leftrightarrow (6x^2-6x^2)+(21x-2x+5x-6x)+(-7+5)=16\\
\Leftrightarrow 18x-2=16\\
\Leftrightarrow 8x=18\\
\Leftrightarrow x=1$
