Chứng minh rằng: \( B = -\frac{1}{3^{4}} + \frac{1}{3^{2}} + \frac{1}{3^{3}} + \ldots + \frac{1}{3^{2020}} + \frac{1}{3^{2021}} < \frac{1}{2} . \)
----- Nội dung ảnh -----
Chứng minh rằng: \( B = -\frac{1}{3^{4}} + \frac{1}{3^{2}} + \frac{1}{3^{3}} + \ldots + \frac{1}{3^{2020}} + \frac{1}{3^{2021}} < \frac{1}{2} . \)