Let \(f(x)=\frac{x^2-4}{2x^2}\), where \(x\ne0\), and let \(F(x)=\frac{1}{2}x+4+\frac{2}{x}\).
a) Show that \(F\) is an antiderivative of \(f\) on each interval \((-\infty, 0)\) and \((0, \infty)\).
b) Find the antiderivative \(G\) of \(f\) on \((0, \infty)\) that satisfies \(G(2)=0\).
c) Find the antiderivative \(H\) of \(f\) on \((-\infty, 0)\) that satisfies \(H(-2)=0\).
d) Explain why the constants in \(G\) and \(H\) do not have to be the same.
Hints
- Simplify \(f\) or differentiate the reciprocal term directly.
- On each connected interval, antiderivatives differ by a constant.
- Use the given point on the positive interval to determine one constant and the point on the negative interval to determine the other.
- Consider the role of the excluded input \(x=0\).
Solution
1. Differentiate \(F\): \(F'(x)=\frac{1}{2}-\frac{2}{x^2}=\frac{x^2-4}{2x^2}=f(x)\). Thus, \(F\) is an antiderivative of \(f\) wherever \(x\ne0\).
2. On \((0, \infty)\), write \(G(x)=F(x)+C\). Since \(F(2)=1+4+1=6\), the condition \(G(2)=0\) gives \(C=-6\). Therefore, \(G(x)=\frac{1}{2}x-2+\frac{2}{x}\).
3. On \((-\infty, 0)\), write \(H(x)=F(x)+D\). Since \(F(-2)=-1+4-1=2\), the condition \(H(-2)=0\) gives \(D=-2\). Therefore, \(H(x)=\frac{1}{2}x+2+\frac{2}{x}\).
4. The domain is split into two disconnected intervals by \(x=0\). A value condition on one interval does not determine the vertical shift on the other interval, so the two constants may differ.
Answer
a) \(F'(x)=f(x)\) for \(x\ne0\).
b) \(G(x)=\frac{1}{2}x-2+\frac{2}{x}\) for \(x>0\)
c) \(H(x)=\frac{1}{2}x+2+\frac{2}{x}\) for \(x<0\)
d) The two domain intervals are disconnected, so each interval can have its own constant of integration.