$[(99$ AND $102$ please$)]$ For the following exercises, find $d^2\frac{y}{d}{x}^2$ at the given point without eliminating the parameter.

$99.$ Determine the concavity of the curve

$x=2t+lnt,y=2t-lnt$

Find the area enclosed by the ellipse

$x=$acos$,y=$bsin$,0<mn>2</mn><mi></mi>$