This equation :
ax+4bx+c=0
must have repeated roots ,so that it becomes tangent
ax2+cx+4b=0
c2-16ab=0
ab ≥0
If ax+by+c=0 is a tangent to xy=4, then find the sign of a and c .
This equation :
ax+4bx+c=0
must have repeated roots ,so that it becomes tangent
ax2+cx+4b=0
c2-16ab=0
ab ≥0