if a number is subtracted from 91, the principal square root of this difference is equal to the number decreased by 1. What's the number?

Question

lgbasallote · Answer

a number is subtracted from 91 so this is 

91 - x

the principal square root of this difference that means positive square root
$$+ \sqrt{91 - x}$$
equal to the number decreased by 1 so

$$+\sqrt{91-x} = x - 1$$
does that give you a clue?

anonymous · Answer

@lgbasallote Might be better to say:
$$|\sqrt{91-x}|$$

lgbasallote · Answer

hmm good point

lgbasallote · Answer

but isnt that $$\pm \sqrt{91-x}?$$

lgbasallote · Answer

$$\pm \sqrt{91-x}*$$

lgbasallote · Answer

for example $$|x| \implies \pm x$$

anonymous · Answer

Yeah, that's true. Guess it's just better to say "positive root". Some may get + beforehand as  only half of an equation.

anonymous · Answer

It is 10 $$91-10=81=9^2$$