Question

nhanh nhaa

Answer 1

Câu 9: \(u_8=u_1+7d\)

=>\(7d=26-\frac13=\frac{77}{3}\)

=>\(d=\frac{11}{3}\)

=>Chọn A

Câu 10: \(S_{n}=\frac{n\cdot\left\lbrack2u_1+\left(n-1\right)\cdot d\right\rbrack}{2}\)

=>\(n\cdot\frac{\left\lbrack2\cdot3+4\left(n-1\right)\right\rbrack}{2}=253\)

=>n(3+2n-2)=253

=>n(2n+1)=253

=>\(2n^2+n-253=0\)

=>\(2n^2-22n+23n-253=0\)

=>(n-11)(2n+23)=0

=>\(\left[\begin{array}{l}n=11\left(nhận\right)\\ n=-\frac{23}{2}\left(loại\right)\end{array}\right.\)

=>n=11

=>Chọn B
Câu 11: A

Câu 12: \(u_6=u_1+5d\)

=>\(5d=27-\left(-3\right)=27+3=30\)

=>d=6

=>Chọn D