Phát biểu sau đúng hay sai, phát biểu lại theo khái niệm "điều kiện đủ": a) Trong mặt phẳng, nếu hai đường thẳng phân biệt cùng vuông góc một đường thẳng thứ ba thì hai đường thẳng ấy song song nhau. b) Nếu hai tam giác bằng nhau thì chúng có diện tích bằng nhau. c) Nếu một số tự nhiên tận cùng là chữ số 5 thì chia hết cho 5. d) Nếu a+b > 5 thì một trong hai số a và b phải dương.

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