Inside Seven Squares

What is the total area of the four outside triangles which are outlined in red in this arrangement of squares inside each other?
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Problem



Seven squares are set inside each other. The centre points of each side of the outer square are joined to make a smaller square inside it and so on.

Image
Inside Seven Squares


The centre square has the area of $1$ (one) square unit.

What is the total area of the four outside triangles which are outlined in red?

This problem is also available in French: Les 7 carrés

Click here for a poster of this problem.