【题目】我们已经知道,完全平方公式可以用平面几何图形的面积来表示,实际上还有一些代数恒等式也可以用这种形式表示,例如:(2a+b)(a+b)=2a~2+3ab+b~2就可以用图2或图3等图形的面积表示.(1)请写出图4所表示的代数恒等式( ).(2)试画出一个几何图形,使它的面积能表示:(a+b)(a+3b)=a~2+4ab+3b~2; [Subject] We already know that the perfect square formula can be expressed by the area of ​​the plane geometry. In fact, some algebraic identities can also be expressed in this form. For example: (2a + b) (a + b) = 2a ~ 2 + 3ab + b ~ 2 can be represented by the area of ​​the graph in Figure 2 or Figure 3. (1) Please write out the algebraic identity () shown in Figure 4. (2) Try to draw a geometry so that its The area can be expressed as: (a + b) (a + 3b) = a ~ 2 + 4ab + 3b ~ 2;