Question 1

What's the difference between an indefinite and a definite integral?

Accepted Answer

An indefinite integral, $\int f(x) \, dx$, gives a general function (a family of curves) and always includes a constant of integration, $+c$. A definite integral, $\int_a^b f(x) \, dx$, has limits and evaluates to a specific number, representing the signed area under the curve between those limits.

Question 2

I forgot to add '+c'. How many marks will I lose?

Accepted Answer

In questions asking for a general solution or an indefinite integral, forgetting '+c' will typically lose you one accuracy mark. For definite integrals, the '+c' is not needed as it cancels out, so you won't lose marks there.

Question 3

My definite integral gave a negative answer. What does this mean?

Accepted Answer

A negative result means that there is more area below the x-axis than above it in the given interval. If the question asks for the 'area', which is a geometric quantity and must be positive, you need to re-evaluate. This usually involves splitting the integral at the roots and taking the absolute value of the parts that are below the axis.

Question 4

How do I know when to use the Trapezium Rule?

Accepted Answer

The question will explicitly tell you to use the trapezium rule. It's used for functions that are difficult or impossible to integrate analytically, or when you are given a table of values instead of a function.

Question 5

How do I integrate something like $\frac{2x+1}{x^2}$?

Accepted Answer

You must first simplify the expression into terms you can integrate using the power rule. Split the fraction: $\frac{2x}{x^2} + \frac{1}{x^2} = \frac{2}{x} + x^{-2}$. Now you can integrate term by term: $\int (\frac{2}{x} + x^{-2}) \, dx = 2\ln|x| + \frac{x^{-1}}{-1} + c = 2\ln|x| - \frac{1}{x} + c$.

Integration

What you need to know

Integration: Summing Up the Slices