The differential equation of all those circles of radius 'a' whose center lies on the X axis will be ________.
1
\(\rm {y} \frac{d y}{d x}=a^2-y^2\)
2
\( \rm y\left(\frac{d y}{d x}+1\right)=a^2 \)
3
\(\rm y^2\left(\frac{d y}{d x}+1\right)=a^2 \)
4
\(\rm y^2\left[1+\left(\frac{d y}{d x}\right)^2\right]=a^2 \)