Hi guys,

So the equation looks like this,

and we're required to use the substitution

Well I do it all and get to

The answer in the back of the book takes the positive square root of this, and I can't see why. It would be easy to shrug it off, since I've done the hard part, but I really would like to know why the positive square root is taken, it's probably something simple I've missed

Thanks

Stonehambey