The sum-difference product formula of 1. trigonometric function: This formula involves the sum-difference product of sine, cosine and tangent functions, which may be difficult for beginners to understand.
2. Chain rule of derivative: This rule is used to find the derivative of compound function. It is necessary to multiply the derivative of inner function with the derivative of outer function, and then take the derivative of inner function. This process may be complicated and needs some practice to master.
3. method of substitution and integration by parts of definite integral: These two methods are both used to calculate definite integral, but their principles and steps are complicated, which requires a certain mathematical foundation to understand.
4. Conditional Probability and Bayes Theorem in Probability Theory: These two concepts both involve the probability calculation of events, but they all need some complicated mathematical formulas, which may be difficult for beginners to understand.
5. Taylor formula in calculus: this formula is used to expand the function into infinite series near a certain point, thus approximately calculating the function value. This formula involves some concepts of higher derivative and infinitesimal, which may be difficult for beginners to understand.