một hình thang vuông có đáy bé bằng 3/5 đáy lớn và có chiều cao bằng 23cm . Người ta mở rộng hình thang bằng cách kéo dài đáy bé cho bằng đáy lớn để được một hình chữ nhật, sau khi mở rộng diện tích hình thang tăng lên 207cm2 .Tìm diện tích hình thang lúc chưa mở rộng

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