Giải thích các bước giải:
\(
\begin{array}{l}
(2x + 1)^5 = \sum\limits_{k = 0}^5 {C_5^k } .2^{5 - k} .x^{5 - k} \\
(a - 2b)^6 = \sum\limits_{i = 0}^6 {C_6^k .a^{6 - i} .( - 2)^i .b^i } \\
(x - \frac{1}{x})^7 = \sum\limits_{m = 0}^7 {C_7^m } .x^{7 - m} .( - 1)^m .\frac{1}{{x^m }} = \sum\limits_{m = 0}^7 {C_7^m } .( - 1)^m .x^{7 - 2m} \\
\end{array}
\)