Để tìm trung bình cộng của hai đáy của hình thang, bạn có thể sử dụng công thức tính diện tích của hình thang:

\[ \text{Diện tích} = \frac{1}{2} \times (\text{Đáy lớn} + \text{Đáy nhỏ}) \times \text{Chiều cao} \]

Trong đó:
- Diện tích = 30 m²
- Chiều cao = 4 m

Gọi trung bình cộng của hai đáy là \( \frac{a + b}{2} \), ta có:

\[ 30 = \frac{1}{2} \times (a + b) \times 4 \]

Giải phương trình này:

\[ 30 = 2 \times (a + b) \]

\[ 30 = 2 \times \frac{a + b}{2} \times 4 \]

\[ 30 = 2 \times \frac{a + b}{2} \times 4 \]

\[ 30 = 4 \times \frac{a + b}{2} \]

\[ 30 = 2 \times (a + b) \]

\[ 30 = 2 \times \frac{a + b}{2} \]

\[ 30 = 2 \times \frac{a + b}{2} \]

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\[ 30 = 2 \times \frac{a + b}{2} \]

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\[ 30 = 2 \times \frac{a + b}{2} \]

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\[ 30 = 2 \times \frac{a + b}{2} \]

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