Câu 16:
\(d=u_2-u_1=6-2=4\)
=>Chọn A
Câu 17:
\(d=u_2-u_1=4-1=3\)
=>Chọn C
Câu 18:
\(d=u_2-u_1=9-3=6\)
=>Chọn D
Câu 19:
\(d=u_2-u_1=8-2=6\)
=>Chọn B
Câu 20:
\(u_6=u_1+5d=2022+5\cdot7=2022+35=2057\)
=>Chọn D
Câu 21:
\(S_{10}=\dfrac{10\cdot\left[2u_1+9\cdot d\right]}{2}=5\cdot\left[2\cdot1+9\cdot2\right]=5\left(2+18\right)=5\cdot20=100\)
=>Chọn B
Câu 22: \(S_n=\dfrac{n\cdot\left[2u_1+\left(n-1\right)\cdot d\right]}{2}\)
=>\(\dfrac{n\cdot\left[2\cdot3+\left(n-1\right)\cdot4\right]}{2}=253\)
=>\(n\left(6+4n-4\right)=253\cdot2=506\)
=>\(4n^2+2n-506=0\)
=>\(\left[{}\begin{matrix}n=11\left(nhận\right)\\n=-\dfrac{23}{2}\left(loại\right)\end{matrix}\right.\)
=>n=11
=>Chọn B
Câu 23:
\(u_n=1-3n\)
=>\(u_1=1-3\cdot1=1-3=-2;d=-3\)
\(S_{10}=\dfrac{10\cdot\left[2\cdot u_1+9\cdot d\right]}{2}\)
\(=5\left[2\cdot\left(-2\right)+9\cdot\left(-3\right)\right]=5\cdot\left[-4-27\right]=-155\)
=>Chọn C
Câu 24: \(S_{14}=\dfrac{14\cdot\left[2\cdot u_1+\left(14-1\right)\cdot d\right]}{2}=7\cdot\left[2\cdot\left(-6\right)+13\cdot4\right]\)
\(=7\cdot\left(-12+52\right)=7\cdot40=280\)
=>Chọn D
