Гурвалжны талбай нь яагаад суурийг өндрөөр үржүүлсний хагастай тэнцүү байдгийг төсөөлцгөөе. 

Intuition for why the area of a triangle is A=12bhA=\dfrac{1}{2}bh

To see why the formula makes sense, drag the dot all the way to the right:
Whoa! You made a rectangle that's twice as big as the triangle! The area of the rectangle is bh=4×5=20bh=4\times 5 = 20 square units, so the area of the triangle is 12bh=12×4×5=10\dfrac 12 bh = \dfrac 12 \times 4 \times 5 = 10 square units.
Key intuition: A triangle is half as big as the rectangle that surrounds it, which is why the area of a triangle is one-half base times height.

Practice problem 1

Practice problem 2: right triangle

Practice problem 3: one vertex is hanging over the side