Concept:
Use property:
\[
\int_0^{a} f(x)\,dx = \int_0^{a} f(a-x)\,dx
\]
Step 1: Let $I = \int_0^{\pi/2} \frac{\sin x - \cos x}{1 + \sin x \cos x} dx$.
Step 2: Replace $x \to \frac{\pi}{2} - x$.
\[
I = \int_0^{\pi/2} \frac{\cos x - \sin x}{1 + \sin x \cos x} dx
\]
Step 3: Add both expressions.
\[
2I = \int_0^{\pi/2} 0 \, dx = 0
\]
Step 4: Solve.
\[
I = 0
\]
Conclusion:
Answer = $0$