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The blue curve diverges from the red curve because it represents a finite polynomial (a partial sum) rather than the infinite series. Finite polynomials have a dominant highest-degree term that dictates their end behavior, causing them to shoot off to positive or negative infinity as $x$ grows large, whereas the infinite series converges to the bounded, periodic cosine function.

Conditions: The blue curve is a finite partial sum (e.g., the first four terms).; The red curve is the target function $\cos x$.; $x$ values are sufficiently large (far from the origin).

The blue curve diverges from the red curve because it represents a finite polynomial (a partial sum) rather than the infinite series. Finite polynomials have a dominant highest-degree term that dictates their end behavior, causing them to shoot off to positive or negative infinity as $x$ grows large, whereas the infinite series converges to the bounded, periodic cosine function.

Conditions: The blue curve is a finite partial sum (e.g., the first four terms).; The red curve is the target function $\cos x$.; $x$ values are sufficiently large (far from the origin).

The video states that a function can be defined by a series to counter the misconception that functions must only be given by simple closed-form algebraic or geometric formulas. By setting $f(x)$ equal to an infinite sum, the series itself serves as the rule that determines the value of the function for any given $x$.

Conditions: The expression is an infinite series.; The equality $f(x) = \text{series}$ is taken as a definition.

The video states that a function can be defined by a series to counter the misconception that functions must only be given by simple closed-form algebraic or geometric formulas. By setting $f(x)$ equal to an infinite sum, the series itself serves as the rule that determines the value of the function for any given $x$.

Conditions: The expression is an infinite series.; The equality $f(x) = \text{series}$ is taken as a definition.