Question

[Euro 2016]

Cho dãy số  được xác định bởi: x1=1;x2=2" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline; float:none; font-family:helvetica neue,helvetica,arial,sans-serif; font-size:14px; line-height:normal; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:0px; position:relative; white-space:nowrap; word-wrap:normal" class="MathJax">$x_1=1;x_2=2$ và xn=−3xn−1+2xn−2+n3+2n+5(n∈N,n≥3)" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline-table; float:none; font-family:helvetica neue,helvetica,arial,sans-serif; font-size:14px; line-height:normal; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:0px; position:relative; white-space:nowrap; word-wrap:normal" class="MathJax">$x_n=-3x_{n-1}+2x_{n-2}+n^3+2n+5\left(n\in N,n\ge 3\right)$. Tính chính xác giá trị của 

\(x^{18+_{x19+x20+x21}}\)x18+x19+x20+x21" role="presentation" style="border:0px; box-sizing:border-box; direction:ltr; display:inline; float:none; font-family:helvetica neue,helvetica,arial,sans-serif; font-size:14px; line-height:normal; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:0px; position:relative; white-space:nowrap; word-wrap:normal" class="MathJax">