52456112
Justify each equation without evaluating the integrals directly. Use properties of definite integrals or symmetry.
a) \(\int_{-5}^{5} (x^3 + x) \, dx = 0\)
b) \(\int_{1}^{4} f(x) \, dx + \int_{4}^{1} f(x) \, dx = 0\)
c) \(\int_{a}^{a} \sqrt{x^2 + 1} \, dx = 0\)
Hints
- Determine how the graph of an odd function behaves under a rotation of \(180^\circ\) about the origin.
- What happens to the sign of a definite integral when its limits are reversed?
- What is the width of the interval when the lower and upper limits are equal?
- Recall that a definite integral represents net signed area.
Solution
1. For a), let \(g(x)=x^3+x\). Since \(g(-x)=(-x)^3+(-x)=-x^3-x=-g(x)\), the integrand is odd. The signed areas on the symmetric interval \([-5, 5]\) cancel, so the integral is \(0\).
2. For b), reversing the limits changes the sign of a definite integral: \(\int_a^b f(x)\,dx=-\int_b^a f(x)\,dx\). Therefore, \(\int_1^4 f(x)\,dx+\int_4^1 f(x)\,dx=\int_1^4 f(x)\,dx-\int_1^4 f(x)\,dx=0\).
3. For c), a definite integral whose lower and upper limits are equal is always \(0\): \(\int_a^a h(x)\,dx=0\).
Answer
a) The integrand is odd and the limits are symmetric about \(0\), so the signed areas cancel.
b) Reversing the limits changes the sign of the integral, so the two integrals cancel.
c) A definite integral with identical limits is \(0\).
