Question

Który wyraz ciągu...

Answer 1

Odpowiedź: 21

[tex]a_n=2n^{2} -53n+300\qquad n\in N_+\\a_n=69\\2n^{2} -53n+300=69\\2n^{2} -53n+231=0\\\Delta=53^2-8\cdot231=961\quad \sqrt{\Delta } =31\\\displaystyle n_1=\frac{53-31}{4} =\frac{22}{4} =\frac{11}{2} \notin N_+\\n_2=\frac{53+31}{4} =\frac{84}{4}=\underline {21}[/tex]