这个观察下就能看出
（1）x=2kπ+π/2,则k无穷时y=1+2kπ+π/2也无穷,故无界
(2)x趋于0时y无穷大
(4)x趋于-1或-2时为无穷大
（3）首先sin(x^2+1)有界,只考虑x^2/(x^4+1)即可
考虑(x^4+1)/x^2=x^2+1/x^2>=2,所以0<=x^2/(x^4+1)<=1/2
两个有界量的乘积一定有界,所以y有界.