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Giải thích các bước giải:

Ta có:

\(\begin{array}{l}
4,\\
a,\\
N = {\left( {x + 3} \right)^3} + {\left( {x – 3} \right)^3} – 2.\left( {x – 1} \right).\left( {{x^2} + x + 1} \right)\\
 = \left( {{x^3} + 9{x^2} + 27x + 27} \right) + \left( {{x^3} – 9{x^2} + 27x – 27} \right) – 2.\left( {{x^3} – {1^3}} \right)\\
 = \left( {2{x^3} + 54x} \right) – \left( {2{x^3} – 2} \right)\\
 = 54x + 2\\
b,\\
N = 2020\\
 \Leftrightarrow 54x + 2 = 2020\\
 \Leftrightarrow 54x = 2018\\
 \Leftrightarrow x = \frac{{1009}}{{27}}\\
3,\\
a,\\
M = {\left( {x + 3} \right)^2} + {\left( {x – 5} \right)^2} – 2x.\left( {x – 4} \right)\\
 = \left( {{x^2} + 6x + 9} \right) + \left( {{x^2} – 10x + 25} \right) – \left( {2{x^2} – 8x} \right)\\
 = \left( {2{x^2} – 4x + 34} \right) – 2{x^2} + 8x\\
 = 4x + 34\\
b,\\
x =  – \frac{1}{4} \Rightarrow M = 4.\left( { – \frac{1}{4}} \right) + 34 = 33\\
c,\\
M = 134\\
 \Leftrightarrow 4x + 34 = 134\\
 \Leftrightarrow 4x = 100\\
 \Leftrightarrow x = 25
\end{array}\)