Two identical triangles overlap with each other as shown. What is the area of the shaded part?

Since the area of (1) + area of (3) = area of (2) + area of (3) , then area of (1) = area of (2) . Thus, the area of the shaded part equals the area of (2) , which is a trapezoid with bases 10 and 10 – 3 = 7 and height 2. The area of such a trapezoid is (10 + 7) × 2 ÷ 2 = 17.