Find the general solutions of (i) (mu−ny)u x
​
+(nx−lu)u y
​
=ly−mx;l,m,n constant. (ii) (x+u)u x
​
+(y+u)u y
​
=0. iii) (x 2
+3y 2
+3u 2
)u x
​
−2xyu y
​
+2xu=0.