1.

If the line ` ax+by+c=0 ` is a tangent to the curve `xy=9,` thenA. ` a gt 0, b gt 0 `B. ` a gt 0, b lt 0 `C. ` a lt 0, b gt 0 `D. ` a lt 0, b lt 0 `

Answer» Correct Answer - A::D
We have,
` xy=9 rArr y=(9)/(x) rArr (dy)/(dx) =-(9)/(x^(2)) " "...(i) `
Let ` ax+by+c=0 ` be a tangent to the curve xy=9 at ` (x_(1),y_(1)). `
Then,
` ((dy)/(dx))_((x_(1)","y_(1))) =" Slope of the line " ax+by+c=0 `
` rArr -(9)/(x_(1)^(2))=(-a)/(b) `
` rArr (x_(1)^(2))/(9) =(b)/(a) `
` rArr (b)/(a) gt 0 `
`rArr` a and b are the same sign.
`rArr (a gt 0 " and " b gt 0 )" or, "(a lt 0 " and " b lt 0 ) `


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