26: (AB;A'C')=(AB;AC)=góc BAC=45 độ
=>Chọn B
27: \(lim\left(-5n^3-n^2+1\right)=lim\left[n^3\left(-5-\dfrac{1}{n}+\dfrac{1}{n^3}\right)\right]\)
lim n^3=+vô cực
\(lim\left(-5-\dfrac{1}{n}+\dfrac{1}{n^3}\right)=-5\)
=>\(lim\left(-5n^3-n^2+1\right)=-\infty\)