Investigate:
Use strips nad fasteners to make a triangle and a quadrilateral.
Try to push inward at any one vertex of the triangle.Do the same to the quadrilateral.
Is the triangle distorted ?Is the quadrilateral distorted?Is the triangle rigid?
Why is it that structures like electric towers make use of triangular shapes and not quadrilaterals?

Use the figure to name: (a) 5 triangles (b) 5 quadrilaterals ( c) 5 angles formed (d) Which two triangles have angleB in common ?

We know that parallelogram is also a quadrilateral. Let us also split such a quadrilateral into two triangles. Find their areas and hence that of the parallelogram. Does this agree with the formula that you know already?

Take any quadrilateral , say ABCD .Divide it into two triangles by drawing a diagonal . You get six angles 1,2,3 ,4,5 and 6 . Use the angles sum property of a triangle and argue how the sum of the measures of angleA,angleB,angleCangleD amounts to 180^(@)+180^(@)=360^(@) .

Let the area of a given triangle ABC be Delta . Points A_1, B_1, and C_1 , are the mid points of the sides BC,CA and AB respectively. Point A_2 is the mid point of CA_1 . Lines C_1A_1 and A A_2 meet the median BB_1 points E and D respectively. If Delta_1 be the area of the quadrilateral A_1A_2DE , using vectors or otherwise find the value of Delta_1/Delta