True or False? In Exercises $73-78$ , determine whether the statement is true or false. If it is false, explain why or give an example that shows it is false.
$$f^{\prime}(x)=g(x),$ then $\int g(x) d x=f(x)+C$$

True or False? In Exercises $73-78$ , determine whether the statement is true or false. If it is false, explain why or give an example that shows it is false.
$\int f(x) g(x) d x=\left(\int f(x) d x\right)\left(\int g(x) d x\right)$

Proof Let $s(x)$ and $c(x)$ be two functions satisfying $s^{\prime}(x)=c(x)$ and $c^{\prime}(x)=-s(x)$ for all $x .$ If $s(0)=0$ and $c(0)=1,$ prove that $[s(x)]^{2}+[c(x)]^{2}=1$

Think About It Find the general solution of $f^{\prime}(x)=-2 x \sin x^{2}$

Suppose $f$ and $g$ are non-constant, differentiable, real- valued functions defined on $(-\infty, \infty) .$ Furthermore, suppose that for each pair of real numbers $x$ and $y$

$f(x+y)=f(x) f(y)-g(x) g(y)$ and
$g(x+y)=f(x) g(y)+g(x) f(y)$

If $$f^{\prime}(0)=0,$ prove that $(f(x))^{2}+(g(x))^{2}=1$ for all $x$$