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

Make a rectangle ABCD having AC as one of its diagonals.

Here, AB = 40 cm and AC = 41 cm

BC =?

By Pythagoras property,

$A{C}^2 = A{B}^2+ B{C}^2$

${41}^2 = {40}^2 + B{C}^2$

$B{C}^2 = {41}^2 – {40}^2$

$B{C}^2 = 1681 – 1600$

$B{C}^2 = 81$

 $BC = \sqrt{81}$

$BC = 9 cm$

Since, the perimeter of the rectangle plot = 2 (length + breadth)

Where, length = 40 cm, breadth = 9 cm

Then,

$= 2(40 + 9)$

$= 2 \times 49$

$= 98 cm$