Today’s official SAT question of the day is a math question that involves two expressions that have been set equal to each other.  This medium-difficulty question looks tougher than it is, as long as we are careful with our signs and do not jump to conclusions. Let’s jump in.

√(x – a) = √(x + b)

First thing: square both sides to get rid of the silly radicals.  Now we are left with:

x – a = x + b

X isn’t in any of our answer choices, so we can make our lives easier by subtracting x from both sides, leaving:

-a = b

One more step to get to our answer, during which we have to be careful with our signs. Let’s get both variables to be positive by moving a to the other side:

0 = b + a

This looks a lot like one of our answer choices:

0 = a + b

Don’t fall into a trap of FOIL-ing when you see a problem like this one; when you square something that’s under a square root, all you’re doing is removing that radical “hat”, so you to NOT need to FOIL anything.  When you find yourself in a situation like this, however, that presents both sides of an equation completely under radicals, you can remove the radicals to make your life easier and get to the answer more quickly.