When you're studying definite integrals in AP Calculus AB, it's important to be careful. Many students make some common mistakes. Here are a few things to watch out for:

1. Misreading the Limits of Integration: Always check your limits! It’s easy to mix them up or forget one completely. The area you calculate relies on these limits. If you're finding the area from point $a$ to point $b$ but you write it as from $b$ to $a$, you could end up with the wrong answer.

2. Ignoring the Sign of the Function: When you integrate a function that goes below the x-axis, remember you're finding net area. If part of the function is negative, the definite integral can also be negative. Finding area doesn’t always mean you’ll get a positive number.

3. Not Applying the Fundamental Theorem of Calculus Correctly: This theorem links differentiation and integration. When you evaluate a definite integral, make sure you're plugging in the upper and lower limits correctly into the antiderivative.

4. Forgetting to Include Units: If you're solving a problem related to physical things, like area or volume, don’t forget the units! It's a small detail, but many students miss it. Units are very important when you want to understand your results.

5. Overlooking the Properties of Integrals: Get to know properties like linearity. For example, when you see the integral of a sum ($\int (f(x) + g(x)) dx = \int f(x) dx + \int g(x) dx$), it can simplify your work. Ignoring these properties might make your calculations harder.

If you keep these tips in mind, you'll do a great job with definite integrals and steer clear of common mistakes!