The area of the parallelogram ABCD is 90 cm². Find area of $∆$BEF.

We know that, If a triangle and a parallelogram are on the same base and between the same parallels, then the area of the triangle is equal to half of the area of the parallelogram.

Here, ΔBEF and parallelogram ABEF are on the same base EF and between the same parallels AB and EF.

Therefore, ar. (ΔBEF) = $\frac{1}{2} \times$ar. (ABEF) = $\frac{1}{2} \times 90 = 45 {\mathrm{cm}}^2$