a) $P={\left(x-10\right)}^2-x\left(x+80\right)$ tại $x=0,87$;

b) $Q=4a^2+8ab+4b^2$ tại $a=65$ và $b=35$;

c) $R=x^3-3x^2+3x-1$ tại $x=101$.

a) $P={\left(x-10\right)}^2-x\left(x+80\right)=x^2-\mathrm{2.10.}x+{10}^2-x^2-80x$

$=\left(x^2-x^2\right)-\left(20x+80x\right)+100=-100x+100$

Với $x=0,87$ ta có: $P=-100.0,87+100=-87+100=13$

b) $Q=4a^2+8ab+4b^2={\left(2a\right)}^2+2.2a.2b+{\left(2b\right)}^2={\left(2a+2b\right)}^2$

Với $a=65$ và $b=35$ ta có: $Q={\left(2.65+2.35\right)}^2={\left(130+70\right)}^2={200}^2=40000$

c) $R=x^3-3x^2+3x-1=x^3-3.x^2.1+3.x{.1}^2-1^3={\left(x-1\right)}^3$

Với $x=101$ ta có: $R={\left(101-1\right)}^3={100}^3=1000000$