sir you told that x and y cannot be zero but in some sums you havw put the value as zero so can you explain how it happens 

in an linear equation

ax+by+c=0

 

if we put x = 0 and y =0 together

then

c=0

 

consider

 

2x+3y+4=0

 if we put x=0, y=0

then

4=0....which is wrong 

 

so x=0 y=0, means c should be equal to 0

so the equation looks like ax+by =0

 

but we can put x=0, alone in any linear equation

and y=0, alone in any linear equation

not together in all equations.