1, the definition of triangle: a figure surrounded by three line segments (the endpoints of every two adjacent line segments are connected or overlapped) is called a triangle. A triangle has three vertices, three angles and three sides. . (For the convenience of expression, the letters A, B and C are used to represent the three vertices of a triangle, which can be represented as triangle ABC or AABC).

2. The height of a triangle is the vertical line from the vertex of the triangle to its opposite side. The line segment between the vertex and the vertical foot is called the height of the triangle, and the opposite side is called the bottom of the triangle. This triangle is only three stories high. Key point: the drawing method of triangle height.

3. Characteristics of triangle: triangle is stable.

4. Trilateral relationship of triangle: the sum of any two sides of triangle is greater than the third side. To judge whether three line segments can form a triangle, we only need to see whether the sum of the smallest two line segments is greater than the sum of the third line segment 5 and the inner angles of the triangle: the sum of the inner angles of the triangle is 180%.

5. Triangle classification: according to classification: acute triangle, pure triangle and straight triangle. Classification by sides: equilateral triangle and isosceles triangle: equilateral triangle (equilateral triangle is a special isosceles three-fish shape.

6. Combination of triangles: two identical triangles can be combined into a parallelogram, two identical Taoist triangles can be combined into a rectangle, and two identical isosceles right triangles can be combined into a square or diamond; Three identical triangles can form a trapezoid. Three identical triangles can form a trapezoid.

7. The formula for calculating the sum of polygons' internal angles is (n-2)x 180 ". Where n is the number of sides, such as: (3-2) x 180 = 180, the sum of internal angles of quadrilateral is: (4-2)x 180=360, and the sum of internal angles of pentagon is: (5-2/kloc-0).

Know the sum and difference of two numbers and find them. Solution: D (sum and difference) 2= decimal, decimal, large number.

First of all, let's be clear: when eight large numbers are changed into decimals, are there eight more large numbers than decimals? No, large numbers should be 8 times more than decimals (that is, 2x8= 16). Only in this way, if you give a decimal number 8, you have an 8, and the two numbers are the same. (Please pay attention to distinguish from the difference between two numbers. )

Solution: (sum -2x8)-2= small number 16 (be careful not to add 8)= large number. (and 2x8)-2= large number-16= decimal.