Tìm các giới hạn saua) <span class="katex-r" data-latex="displaystyle A=lim _{n rightrow+∞} ac{3+3^2+3^3+cdots+3^n}{2^n+3^n}"></span>;b) <span class="katex-r" data-latex="displaystyle B=lim _{x rightrow 0} ac{e^x-st{2} s∈ (x+ac{π}{4}right)}{x^2}"></span>.