The diagonal of the rectangle is 41 cm and the side is 40 cm. Find the area of the rectangle.

Given: rectangle, diagonal = 41 cm, side = 40.
Find: area -?
Decision:
Since we are given a diagonal, it means that it divides the rectangle into 2 right-angled triangles.
Therefore, from the triangle we can find the side by the Pythagorean theorem:
x ^ 2 = 41 ^ 2 – 40 ^ 2.
x ^ 2 = 1681 – 1600.
x ^ 2 = 81.
x = 9.
We found the unknown side, but to find the area you need 9 * 40, and this will be equal to 360.
Answer: 360.