A parallelogram has sides 60 m and 40 m and one of its diagonals is 80 m long. Its area is

$$\eqalign{ & S\left( {\Delta ABD} \right) = \frac{{60 + 80 + 40}}{2} = 90 \cr & {\text{ar }}\Delta ABD = \sqrt {90\left( {90 - 80} \right)\left( {90 - 60} \right)\left( {90 - 40} \right)} \cr & = \sqrt {90 \times 10 \times 30 \times 50} \cr & = 300\sqrt {15} {\text{ }}{{\text{m}}^2} \cr & {\text{ar }}\square ABCD = 2 \times {\text{ar }}\Delta ABD = 600\sqrt {15} {\text{ }}{{\text{m}}^2} \cr} $$