Máy bơm thứ I để giá bán 1500000 đồng, điện năng tiêu thụ 1, 2kW/giờ, máy thứ II giá bán 2000000 đồng, điện năng tiêu thụ 2kW/giờ. Trung bình mỗi ngày phải sử dụng máy 1 giờ. a) Với x là số ngày sử dụng, y là chi phí bỏ ra. Viết biểu thức liên hệ giữa x và y, giả sử mỗi kW điện đều có giá 1500 đồng. b) Hỏi trong một tháng (30 ngày) thì máy nào đạt hiệu quả kinh tế hơn. Biết tiền điện 50kw đầu tính với giá 1484 đồng/1kw, 50kw tiếp theo có giá 1533 đồng/1kw.

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