[數學]菱形面積

Area of a rhombus ≡ s2 sin A ≡ (d1 x d2)/2
Note that it should be (d1 x d2)/2, not (d1 + d2)/2 as you wrote above.
Generally, s2 is not correct, except (and, only except) the case that the rhombus is a square (i.e. when A = π/2).