Mathematics in America is not that difficult, it mainly depends on what you study. For example, the most basic calculus in universities, American universities generally have several kinds of calculus. For example, calculus for business students is relatively simple, so it is enough to take derivatives and integrals. Then the calculus of science and engineering is a little more difficult, and there will be some proofs in it. It is usually called "high number" in China, and then the calculus of mathematics department.