7. The range of the function $f(x)=\frac{\sin ^{2} x}{1+\cos ^{2} x}+\frac{\cos ^{2} x}{1+\sin ^{2} x}$ is
7. $\left[\frac{2}{3}, 1\right]$.
$f(x)=\frac{\sin ^{2} x}{1+\cos ^{2} x}+\frac{\cos ^{2} x}{1+\sin ^{2} x}=\frac{24}{8+\sin ^{2} 2 x}-2$, and $\sin ^{2} 2 x \in[0,1]$, so $f(x) \in$ $\left[\frac{2}{3}, 1\right]$.