a) thể tích của hình chữ nhật là:\(x.(x+1)(x-1)=x.(x^2-1)=x^3-x\)

b)thể tích của hình chữ nhật tại:\(x=4là:4^3-4=6043-4=60\)

\(2x^4-3x^3-3x^2+6x-2:x^2-2\)

\(=2x^2-3x+1\)

5x(4x2−2x+1)−2x(10x2−5x+2)=−36 

\(5 x . 4 x^{2} + 5 x . \left(\right. - 2 x \left.\right) + 5 x . 1 + \left(\right. - 2 x \left.\right) . 10 x^{2} + \left(\right. - 2 x \left.\right) . \left(\right. - 5 x \left.\right) + \left(\right. - 2 x \left.\right) . 2 = - 36\) 

\(20 x^{3} + \left(\right. - 10 x^{2} \left.\right) + 5 x + \left(\right. - 20 x^{3} \left.\right) + 10 x^{2} + \left(\right. - 4 x \left.\right) = - 36\)

\(\left(\right. 20 x^{3} - 20 x^{3} \left.\right) + \left(\right. - 10 x^{2} + 10 x^{2} \left.\right) + \left(\right. 5 x - 4 x \left.\right) = - 36\)

\(x = - 36\)

Vậy \(x = - 36\).

a)\(P\left(x\right)+Q\left(x\right)=\left(x^4-5x^3+4x-5\right)+\left(-x^4+3x^2+2x+1\right)\)

\(=x^4-5x^3+4x-5+\left(-x^4\right)+3x^2+2x+1\)

\(=\left(x^4-x^4\right)-5x^3+3x^2+\left(4x+2x\right)-\left(5-1\right)\)

\(=-5x^3+3x^2+6x-4\)

b)\(R\left(x\right)=P\left(x\right)-Q\left(x\right)\)

\(=x^4-5x^3+4x-5+x^4-3x^2-2x-1\)

\(=\left(x^4+x^4\right)-5x^3-3x^2+\left(4x-2x\right)+\left(5-1\right)\) \(=2x^4-5x^3-3x^2+4x+4\)

\(\rArr R\left(x\right)=2x^4-5x^3-3x^2+4x+4\)