### Integration Formulas of Trigonometric Functions PDF – Riemann Integral

The Riemann integral is defined in terms of Riemann sums of functions with respect to tagged partitions of an interval. A tagged partition of a closed interval [a, b] on the real line is a finite sequence

- {\displaystyle a=x_{0}\leq t_{1}\leq x_{1}\leq t_{2}\leq x_{2}\leq \cdots \leq x_{n-1}\leq t_{n}\leq x_{n}=b.\,\!}

This partitions the interval [*a*, *b*] into n sub-intervals [*x*_{i−1}, *x*_{i}] indexed by i, each of which is “tagged” with a distinguished point *t*_{i} ∈ [*x*_{i−1}, *x*_{i}]. A *Riemann sum* of a function f with respect to such a tagged partition is defined as

- {\displaystyle \sum _{i=1}^{n}f(t_{i})\,\Delta _{i};}

thus each term of the sum is the area of a rectangle with height equal to the function value at the distinguished point of the given sub-interval, and width the same as the width of sub-interval, Δ_{i} = *x*_{i}−*x*_{i−1}. The *mesh* of such a tagged partition is the width of the largest sub-interval formed by the partition, max_{i=1…n} Δ_{i}. The *Riemann integral* of a function f over the interval [*a*, *b*] is equal to S if:

- For all {\displaystyle \varepsilon >0} there exists {\displaystyle \delta >0} such that, for any tagged partition {\displaystyle [a,b]} with mesh less than {\displaystyle \delta },

- {\displaystyle \left|S-\sum _{i=1}^{n}f(t_{i})\,\Delta _{i}\right|<\varepsilon .}

When the chosen tags give the maximum (respectively, minimum) value of each interval, the Riemann sum becomes an upper (respectively, lower) Darboux sum, suggesting the close connection between the Riemann integral and the Darboux integral.

### Lebesgue integral

It is often of interest, both in theory and applications, to be able to pass to the limit under the integral. For instance, a sequence of functions can frequently be constructed that approximate, in a suitable sense, the solution to a problem. Then the integral of the solution function should be the limit of the integrals of the approximations. However, many functions that can be obtained as limits are not Riemann-integrable, and so such limit theorems do not hold with the Riemann integral. Therefore, it is of great importance to have a definition of the integral that allows a wider class of functions to be integrated.

Such an integral is the Lebesgue integral, which exploits the following fact to enlarge the class of integrable functions: if the values of a function are rearranged over the domain, the integral of a function should remain the same. Thus Henri Lebesgue introduced the integral bearing his name, explaining this integral thus in a letter to Paul Monte.

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