giúp với ak ----- Nội dung ảnh ----- Bài 9(VD) Cho \( A=1-\frac{3}{4}+\left(\frac{3}{4}\right)^{2}-\left(\frac{3}{4}\right)^{3}+\left(\frac{3}{4}\right)^{4}-\left(\frac{3}{4}\right)^{5}-\ldots -\left(\frac{3}{4}\right)^{2019}+\left(\frac{3}{4}\right)^{2020} \)
