$\"\"$

The rhombus with a side is $\"\"$ .

And included angle is $\"\"$ .

Draw the rhombus with above specifications:

$\"\"$

A line drawn from $\"\"$ to $\"\"$ creates two congruent triangles.

Consider the $\"\"$ :

The area of triangle is $\"\"$ .

$\"\"$ .

Observe the figure ,the total area of rhombus is the sum of the areas of $\"\"$ and $\"\"$ .

And the areas of $\"\"$ and $\"\"$ are equal.

Area of rhombus is $\"\"$ .

Therefore rhombus area is $\"\"$ .

$\"\"$

$\"\"$ .