Algebra and right triangles

[AlaskaR]

Please explain your answer. Please explain the use of A referred to in an equation.

 

[Dr.Peterson]

I presume \(A_{something}\) means the area of the something.

Express each smaller triangle as a numerical area, knowing the whole parallelogram is 4. Think about the proportions in which AB and AD are divided.

There are, of course, no right triangles here.