Just revising, and came across a few limits I couldn't remember how to prove.

1. lim(x,y)-->(0,0) (cosx-1+x^2/2)/(x^4-y^4)

2. lim(x,y)-->(0,0) (y-2x+sin2x)/(x^3+y)

And these, which are supposed to use sandwich rule

1. 7x^2*y^2/(x^2+2y^4)

2. 3y*x^2/(x^2+y^2)

Thanks