Are you allowed to use L'Hospital's Rule? If so,
You can use the same approach for the other two problems.
Neither do I. But I can help you.
Use L'hopital's Rule. If you have a limit that approaches 0, the function is a fraction, and the solution comes out undefined, get the derivative of the numerator, and the derivative of the denominator, and you should be able to solve it. I'll do number one:
lim x-->0 x^2/1-cosx
becomes:
lim x-->0 2x/sinx
becomes:
lim x-->0 2/cosx
becomes:
2/1
and you can repeat it as much as you like as long as the limit approaches zero.
1. Either use l'Hopital's Rule or substitute the Maclaurin series for cos x and simplify. Or use a useful trig limit found here: World Web Math: Useful Trig Limits
2. Note that it can be writen as . Now use a useful trig limit.
3. Note that it can be written as . Now use a useful trig limit.