Một kiến trúc sư thiết kế một hội trường với 15 ghế ngồi ở hàng thứ nhất, 18 ghế ngồi ở hàng thứ hai, 21 ghế ngồi ở hàng thứ ba và cứ như vậy (số ghế ngồi ở hàng sau nhiều hơn 3 ghế so với số ghế ngồi ở hàng liền trước nó). Nếu muốn hội trường đó có số sức chứa ít nhất 870 ghế ngồi thì kiến trúc sư phải thiết kế tối thiểu bao nhiêu hàng ghế.
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Số ghế ở các hàng tạo thành một cấp só cộng có \(u_1=15\) và công sai \(d=3\).
Giả sử hội trường có \(n\) hàng ghế \(n \in \mathbb{N}^*\).
Tổng số ghế có trong hội trường là:
\({{S}_{n}}=\frac{\left[ 2{{u}_{1}}+(n-1)d \right].n}{2}=\frac{[2.15+(n-1).3]n}{2}=\frac{3{{n}^{2}}+27n}{2}.\)
Để hội trường đó có số sức chứa ít nhất 870 ghế ngồi thì \(S_n \geq 870\)
\(\Leftrightarrow \frac{3 n^2+27 n}{2} \geq 870 \Leftrightarrow n^2+9 n-580 \geq 0 \Leftrightarrow\left[\begin{array}{l}n \geq 20 \\n \leq-29\end{array} \right.\)
Vậy kiến trúc sư phải thiết kế tối thiểu 20 hàng ghế.
Với cấu trúc mới lạ và đa dạng, Đề Thi Tham Khảo Đánh Giá Năng Lực Năm 2025 – ĐHQG Hà Nội – Đề Số 4 giúp thí sinh phát huy tối đa khả năng Giải Quyết Vấn Đề, Sáng Tạo và Tư Duy Định Lượng. Bài thi bao gồm ba phần chính: Toán Học Và Xử Lí Số Liệu, Văn Học - Ngôn Ngữ, Khoa Học/Tiếng Anh. Mỗi phần thi đều được thiết kế với các dạng câu hỏi phong phú, không chỉ đánh giá kiến thức lý thuyết mà còn kiểm tra khả năng phân tích dữ liệu, xử lý thông tin và vận dụng thực tế. Đây là lựa chọn hoàn hảo cho học sinh muốn tự tin chinh phục kỳ thi đánh giá năng lực.
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Đâu là chức năng của loa?
Đâu là ý đúng: Nước có phát thải khí nhà kính lớn nhất trên thế giới là?
Hoạt động của chủ thể trung gian giúp ích gì cho người sản xuất và người tiêu dùng?
Câu thơ “Tùng cúc lưỡng khai tha nhật lệ” có thể hiểu là:
