You wouldn't count the fuse section as part of the wing area. Although it technically does provide a small amount of lift, it isn't considered part of the wing.
So the formula is:

WingArea = ([(RootChord + TipChord) / 2] * Length) * 2

Which simplifies to:

WingArea = Length * (RootChord + TipChord)


Note:
Length = distance perpendicular to fuse to tip. NOT (trailing edge length + leading edge length) / 2