Question

$x+y=k$ will be a tangent to $16x^2+9y^2=144$ if $k=$

• A. $\sqrt{7}$
• B. $25$
• C. $5$
• D. $7$

Answer 1

The correct option is C

$5$

For line $y=-x+k$ to be tangent to$\frac{x^2}{9}+\frac{y^2}{16}=1$
If
$c^2=a^2{m}^2+b^2$

$k^2=9+16=25\Rightarrow k=\pm 5$