Bài 9:
3: \(C=2x\left(6x-4\right)-5x^2\left(8+3x\right)+2\left(5x^2+4x+1\right)+3x^2\left(5x+6\right)\)
\(=12x^2-8x-40x^2-15x^3+15x^3+18x^2+2\cdot\left(5x^2+4x+1\right)\)
\(=-10x^2-8x+10x^2+8x+2=2\)
5: \(\left(x^2+x+1\right)\left(2x^2-x+3\right)\)
\(=2x^4-x^3+3x^2+2x^3-x^2+3x+2x^2-x+3\)
\(=2x^4+x^3+4x^2+2x+3\)
\(E=\left(x^2+x+1\right)\left(2x^2-x+3\right)-\left(2x^4+x^3+4x^2-x-2\right)-\left(3x-5\right)-3\)
\(=2x^4+x^3+4x^2+2x+3-2x^4-x^3-4x^2+x+2-\left(3x-5\right)-3\)
\(=3x+5-3x+5\)-3
=10-3=7
Bài 10:
2: 4(x+2)-7(2x-1)+9(3x-4)=30
=>\(4x+8-14x+7+27x-36=30\)
=>17x-21=30
=>17x=51
=>x=3
3: \(2\left(5x-8\right)-3\left(4x-5\right)=4\left(3x-4\right)+11\)
=>10x-16-12x+15=12x-16+11
=>-2x-1=12x-5
=>-14x=-5+1=-4
=>\(x=\dfrac{4}{14}=\dfrac{2}{7}\)
4: \(3x\left(x-2\right)-3\left(x^2+1\right)=x^2+1-x\left(x-2\right)\)
=>\(3x^2-6x-3x^2-3=x^2+1-x^2+2x\)
=>-6x-3=2x+1
=>-8x=4
=>\(x=-\dfrac{1}{2}\)
5: \(5\left(3x+5\right)-4\left(2x-3\right)=5x+3\left(3x+12\right)\)
=>15x+25-8x+12=5x+9x+36
=>7x+37=14x+36
=>-7x=-1
=>\(x=\dfrac{1}{7}\)
6: \(\left(7x+7\right)+3x\left(2x-1\right)-2x\left(3x+15\right)=-4\)
=>\(7x+7+6x^2-3x-6x^2-30x=-4\)
=>-26x=-11
=>\(x=\dfrac{11}{26}\)
