Câu 1:
\(\left\{\begin{matrix} U_{2}-U_{3}+U_{5}=10
& & \\ U_{4}+U_{6}=26
& &
\end{matrix}\right.\)
\(\left\{\begin{matrix} U_{1}+d-U_{1}-2d+U_{1}+4d=10 & & \\ U_{1}+3d+U_{1}+5d=26 & & \end{matrix}\right.\)
\(\left\{\begin{matrix} U_{1}+3d=10
& & \\ 2U_{1}+8d=26
& &
\end{matrix}\right.\)
\(\left\{\begin{matrix} U_{1}=1 & & \\ d=3 & & \end{matrix}\right.\)
b. \(U_{10}=U_{1}+9d=1+9.3=28\)
\(S_{10}=nU_{1}+\dfrac{(n-1)d}{2}=10.1+\dfrac{9.3}{2}=23,5\)
Câu 2:
a. \(U_{1}=2.1=2\)
\(U_{2}=2.2=4\)
\(q=\dfrac{U_{2}}{U_{1}}=\dfrac{4}{2}=2\)
b. \(U_{5}=U_{1}.q^{4}=2.2^{4}=32\)
\(S_{5}=\dfrac{U_{1}(1-q^{5})}{1-q}=\dfrac{2.(1-2^{5})}{1-2}=62\)
