Câu hỏi của harumi05

Question

Tìm x biết:

a) $\left(3x-1\right)\left(2x+7\right)-\left(x+1\right)\left(6x+5\right)=x+2-\left(x-5\right)$

b) $\left(x+1\right)\left(x+2\right)\left(x+5\right)-x^3-8x^2=27$

Yukru · Accepted Answer

a) $\left(3x-1\right)\left(2x+7\right)-\left(x+1\right)\left(6x+5\right)=x+2-\left(x-5\right)$

$\Rightarrow\left(6x^2+21x-2x-7\right)-\left(6x^2+5x+6x+5\right)=x+2-x+5$

$\Rightarrow6x^2+19x-7-6x^2-11x-5=7$

$\Rightarrow8x-12=7$

$\Rightarrow8x=7+12$

$\Rightarrow8x=19$

$\Rightarrow x=\dfrac{19}{8}$

b) $\left(x+1\right)\left(x+2\right)\left(x+5\right)-x^3-8x^2=27$

$\Rightarrow\left(x^2+3x+2\right)\left(x+5\right)-x^3-8x^2=27$

$\Rightarrow x^3+3x^2+2x+5x^2+15x+10-x^3-8x^2=27$

$\Rightarrow17x+10=27$

$\Rightarrow17x=17$

$\Rightarrow x=1$Câu trả lời: a) $\left(3x-1\right)\left(2x+7\right)-\left(x+1\right)\left(6x+5\right)=x+2-\left(x-5\right)$

$\Rightarrow\left(6x^2+21x-2x-7\right)-\left(6x^2+5x+6x+5\right)=x+2-x+5$

$\Rightarrow6x^2+19x-7-6x^2-11x-5=7$

$\Rightarrow8x-12=7$

$\Rightarrow8x=7+12$

$\Rightarrow8x=19$

$\Rightarrow x=\dfrac{19}{8}$

b) $\left(x+1\right)\left(x+2\right)\left(x+5\right)-x^3-8x^2=27$

$\Rightarrow\left(x^2+3x+2\right)\left(x+5\right)-x^3-8x^2=27$

$\Rightarrow x^3+3x^2+2x+5x^2+15x+10-x^3-8x^2=27$

$\Rightarrow17x+10=27$

$\Rightarrow17x=17$

$\Rightarrow x=1$

Bài 1: Nhân đơn thức với đa thức