5x(4x2−2x+1)−2x(10x2−5x+2)=−36 

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

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

\(x=-36\)

\(\) \(\)  \(\Rightarrow x=-36\).

a) \(P\left(\right.x\left.\right)+Q\left(x\right.\left.\right)\) 

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

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

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

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

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

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

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

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

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