>>12
sqr() is a function. it returns only one value for any given input.
>>9
would be a half circle.
y = x^2 has two solutions. x = sqr(y) and -sqr(y). you have to include the -sqr() err... manually when you solve for x since the square root function only returns a positive number.
>>2
>>3
don't forget limits. derivatives and integrals by definition are both limits of something.
http://en.wikipedia.org/wiki/Limit_(mathematics)