A parallelogram has sides 60 m and 40 m and one of the diagonal is 80 m long. Then its area is?

AB = 60 m, BC = 40 m and AC = 80 m
∴ s = (60 + 40 + 80 ) / 2 m = 90 m
(s-a) = 90 - 60 = 30 m,
(s-b) = 90 - 40 = 50 m and
(s-c) = 90 - 80 = 10 m
∴ Area of Δ ABC =
√s(s-a)(s-b)(s-c)
= √90 x 30 x 50 x 10 m²
= 300√15 m²
∴ Area of parallelogram ABCD = 2 x area of Δ ABC
= 600√15 m²