Câu 3:
a: \(u_2=4\cdot u_1-9=4\cdot8-9=32-9=23\)
\(v_1=u_1-3=8-3=5\)
=>Đúng
b: \(v_2=u_2-3=23-3=20\)
Vì \(\dfrac{v_2}{v_1}=\dfrac{20}{5}=4\ne3\)
nên Sai
c: \(u_3=4\cdot u_2-9=4\cdot23-9=92-9=85\)
\(v_3=u_3-3=85-3=82\)
\(5\cdot\left(-3\right)^{3-1}=5\cdot\left(-3\right)^2=45\ne82\)
=>Sai
Câu 4:
a: \(u_1=\dfrac{1-3^1}{2\cdot3^{1-2}}=\dfrac{1-3}{2\cdot3^{-1}}=-3\)
=>Đúng
b: \(S_2=\dfrac{1-3^2}{2\cdot3^{2-2}}=\dfrac{1-9}{2}=-\dfrac{8}{2}=-4\)
=>\(u_1+u_2=-4\)
=>\(u_2=-4-u_1=-4-\left(-3\right)=-4+3=-1\)
=>Sai
c: \(u_1=\dfrac{1}{3^{1-2}}=\dfrac{1}{3^{-1}}=1:\dfrac{1}{3}=3\ne-3\)
=>Sai
d:
\(S_3=\dfrac{1-3^3}{2\cdot3^{3-2}}=\dfrac{1-27}{2\cdot3}=\dfrac{-26}{3}\)
=>\(u_1+u_2+u_3=S_3\)
=>\(-3+\left(-1\right)+u_3=-\dfrac{26}{3}\)
=>\(u_3=-\dfrac{26}{3}+4=-\dfrac{14}{3}\)
Vì \(u_2^2\ne u_1\cdot u_3\)
nên đây không là cấp số nhân
=>Sai
