sub(下标元素)

 HTML <sub> 元素用于指定应显示为下标的行内文本，这完全是出于排版的原因。下标通常使用较小的文本以较低的基线显示。

水的化学式是H₂O，即一个水分子中含有2个氢原子和1个氧原子。

四氧化三铁是一种无机物，其化学式为Fe₃O₄，为具有磁性的黑色晶体，故又称为磁性氧化铁。

H₂O is essential for life, and its molecular structure is represented as H₂. In physics, Einstein's famous equation E = mc² revolutionized our understanding of energy. The equation of a parabola in standard form is y = ax² + bx + c. For chemical reactions, consider the formula for glucose: C₆H₁₂O₆. Meanwhile, in mathematics, summation is expressed as ∑_i=1ⁿ i² = n(n+1)(2n+1) / 6. Advanced topics like Schrödinger's equation involve ψ(x,t) = e^{i(kx - ω t)}, where i is the imaginary unit and ψ = 2π f.

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