Question

Find the perimeter of the rectangle whose length is $40$ $cm$ and a diagonal is $41$ $cm$.

Answer 1

In a rectangle, all interior angles are of $90\degree$ measure. 
Therefore, Pythagoras theorem can be applied here.

$(41)^2= (40)^2 + x^2$

$\Rightarrow$$1681$ = $1600 + x$

$2 \times2= 1681 -1600$ = $81$ 
$x = 9$ $cm$

$Perimeter = 2(Length + Breadth)$

= $2(x + 40)$

= $2 (9 + 40)$

= $98$ $cm$