恒等式

こうとうしき

noun

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identity(mathematics)

恒	constancy, always
等	etc., and so forth, class (first), quality, equal, similar
式	style, ceremony, rite, function, method, system, form, expression

もんだい

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こうとうしき

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よしき

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This problem is an equality of k so we first solve the assigned equation for k