Hey can someone help understanding this topic! U sub versus no U sub!

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I want to understand how these two integrals are equal say we integrate $$\int(x-10)dx$$ if we do it normally we will get $$\frac12x^2-10x$$ however we can also u-sub to save time $$\int u du$$ where u is x-10 and get $$\frac12(x-10)^2$$ How are these equivalent