Why does sqrt(25-x^2) need trigonometric Integration?

The graph of the equation ##y = \sqrt{25-x^2}## is the upper semi-circle of radius 5, centered at (0,0). So, when you integrate y from x = a to x = b you are computing the area under part of a semi-circle, and that suggests that things like trigonometric functions will be needed.

 

I usually tell my students that trig sub is something you want to think about trying when you have a quadratic expression in a "bad place"; under a radical or in a denominator. Of course you'd want to look for an easy u-sub or algebraic rearrangement first, but that seems to be a good "trigger". Especially for textbook-style problems in a standard calc course.

 

It doesn't "require" trigonometric substitution.
You might, for example, to arbitrary degree of accuracy, use numerical integration instead.

That being said, using trigonometric integration is very..clever.

 

Every time I ever see something that looks like ##y = \sqrt{a^2 ± b^2}## the first things that pop into my head is "Pythagorean theorem" and "triangle," and in the context of integration "trig substitution" comes immediately after. For me, I remember all the trig I used in geometry of right triangles, and that triggers me to think trig substitution (sorry bout the pun ;) ).

Quick question. Do you mean more specifically, the quantity being raised to the 1/2 power is not the DUMMY VARIABLE of the integration? (which is x in this case)

If it were
$$\int x^n dx $$

we'd have the dummy variable (x) being raised to the power and could go ahead and use the rule. Is that right?