Answer:
A = a*b*Cos(θ-90º)
Step-by-step explanation:
Area of a parallelogram : A = b*h, where h is the height
h form 90º angle with b.
h = a*Cos (θ-90º) in a sub-triangle formed by h, a and a*Sin(θ-90º)
In the Pythagoearn identity for this sub-triangle:
(a*Sin(θ-90º))² + (a*Cos(θ-90º))² = a²
(a*Sin(θ-90º))² + h² = a²
h² = a² (1 - Sin² (θ-90ª))
h² = a² (Cos²(θ-90º)
h = a (Cos (θ-90º))
