相反相成 Opposites complement each other
Explanation
指表面上看起来对立的两个事物，实际上却相互依赖、相互促进，共同构成一个完整的整体。
Refers to two things that appear to be opposite on the surface, but in fact depend on and promote each other, together forming a complete whole.
Origin Story
传说中，太极图以阴阳鱼的形态展现了相反相成的道理。黑白两鱼相互缠绕，看似对立，实则互为一体，互相依存，共同构成一个完整的圆形。黑色代表阴，白色代表阳，阴中有阳，阳中有阴，阴阳互根互用，生生不息，这便是宇宙万物的运行规律。在一个小山村里，住着一位年迈的农夫和他的孙子。农夫以种植水稻为生，他发现，水稻的生长需要充足的阳光和水分，但如果阳光过于强烈，水分不足，水稻就会枯萎；如果水分过多，阳光不足，水稻也会生长不良。农夫和孙子经过反复的试验，最终总结出水稻的最佳生长条件，既需要适量的阳光，也需要适量的水分。阳光和水分看似对立，却互相依存，缺一不可。这就好比阴阳相成，相反相成，只有平衡和谐，才能达到最好的效果。
Legend says the Taijitu, with its shape of Yin and Yang fish, shows the principle of opposite complementarity. The two fish, black and white, intertwine, seemingly opposite but actually forming a whole, interdependent, creating a complete circle. Black represents yin, white represents yang; yin contains yang, yang contains yin, they are interdependent, creating a continuous cycle, this is the running law of all things in the universe. In a small mountain village, there lived an old farmer and his grandson. The farmer made a living by growing rice. He found that the growth of rice needs sufficient sunlight and water, but if the sunlight is too strong and the water is insufficient, the rice will wither; if the water is too much and the sunlight is insufficient, the rice will also grow poorly. After repeated experiments, the farmer and his grandson finally summarized the best growing conditions for rice, which need an appropriate amount of sunlight and water. Sunlight and water seem to be opposed to each other, but they depend on each other and are indispensable. This is similar to the complementarity of yin and yang, the complementarity of opposites, only balance and harmony can achieve the best results.
Usage
常用来形容看似矛盾的两个事物，实际上却互相依存，互相促进。
Often used to describe two things that seem contradictory but actually depend on and promote each other.
Examples
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阴阳相生相克，体现了相反相成的道理。
yīn yáng xiāng shēng xiāng kè, tiǎn xian le xiāng fǎn xiāng chéng de dào lǐ。
The generation and restraint of yin and yang embody the principle that opposites complement each other.
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矛盾的双方，在一定条件下可以互相转化，相反相成。
máo dùn de shuāng fāng, zài yī dìng tiáo jiàn xià kě yǐ hù xiāng zhuǎn huà, xiāng fǎn xiāng chéng。
Contradictory aspects can transform into each other under certain conditions, complementing each other in opposition.