( 10×8÷2- 10×3÷2)×2

=(80÷2-30÷2)×2

=(40- 15)×2

=25×2

=50 square centimeters

Because there are two triangles with equal base and height (base = 10cm, height =8cm) in a right-angled trapezoid, the area of the two shadow triangles on the left and right sides is equal to the area of the two triangles with equal base and height minus the area of the triangle with middle base = 10cm and height =3cm.