Đáp án:
$\begin{array}{l} A = 1.3 + 3.5 + 5.7 + ... + 49.51\\ \Rightarrow 6A = 6\left( {1.3 + 3.5 + 5.7 + ... + 49.51} \right)\\ \Rightarrow 6A = 1.3.6 + 3.5.6 + 5.6.7 + ... + 49.51.6\\ \Rightarrow 6A = 1.3\left( {5 + 1} \right) + 3.5\left( {7 - 1} \right) + 5.7\left( {9 - 3} \right) + ... + 49.51\left( {53 - 47} \right)\\ \Rightarrow 6A = 1.3.5 + 1.3.1 + 3.5.7 - 1.3.5 + 5.7.9 - 3.5.7 + ... + 49.51.53 - 47.49.51\\ \Rightarrow 6A = 49.51.53 + 1.3.1\\ \Rightarrow A = 132450:6\\ \Rightarrow A = 22075 \end{array}$
CHÚC EM HỌC TỐT!