How is $\int_0^{\pi/2} \cos x\,dx$ computed from the cosine power series in this clip?

The DCT justifies exchanging the definite integral over $[a,b]$ with the infinite summation, allowing term-by-term integration. Specifically, it establishes that $\int_a^b \sum f_n(x)\,dx = \sum \int_a^b f_n(x)\,dx$.

Conditions: The previously stated convergence, domination, and finite-integral conditions are assumed to hold.; The summation is over the sequence of functions $f_n$.; The integration is over the closed interval $[a,b]$.

The three hypotheses are: (1) a sequence of functions $f_n$ converges to a function $f$ on the interval $[a,b]$; (2) there exists a dominating function $g$ such that $|g(x)| \ge |f_n(x)|$ for all $n$ and all $x \in [a,b]$; and (3) the integral of the dominating function over the interval is finite, meaning $\int_a^b g(x)\,dx < \infty$.

Conditions: The theorem is presented in a calculus-oriented form rather than a full measure-theoretic proof.; The interval $[a,b]$ is a closed, bounded real interval.

The sum and integral can be interchanged because the Dominated Convergence Theorem applies. The terms of the cosine power series are bounded in magnitude by $1$ on the interval $[0,\pi/2]$, making the constant function $g(x)=1$ a valid dominating function with a finite integral over the interval.

Conditions: The integration interval is $[0,\pi/2]$.; The series terms are $\frac{(-1)^n}{(2n)!}x^{2n}$.; The dominating function is $g(x)=1$.

The example is included to demonstrate that the domination hypothesis alone is insufficient if the dominating function's integral diverges. It serves as a warning that if the finite-integral condition is not met, the desired interchange of integral and sum may fail.

Conditions: The example is presented as a warning illustration on the interval $[0,\infty)$.; The function $e^{2x}$ dominates $e^x$, but $\int_0^{\infty} e^{2x}\,dx$ diverges.