Mathematically, there are several examples to solve the binary linear equation and the projective theorem in the third grade.

Mixed addition and subtraction substitution method for binary linear equation (1). Example 1,13x+14y = 41(1)14x+13y = 40 (2) solution: x-y = +14y = 4113y-13+14y = 4127y = 54y = 2 Substitute y = 2 into (3) to get x=/kloc. (2) method of substitution method of substitution is another method of binary linear equation, that is to say, one equation is brought into another equation, for example, x+y=590 y+20=90%x, that is, x+90%x-20=590 cases 2, (X+5)+(Y-). Y-4=n The original equation can be written as m+n=8 m-n=4, and the solution is m=6 and n=2, so x+5=6 and y-4=2, so x= 1 and y=6. Features: The two equations contain the same algebraic expressions, such as x+5, y-4, etc. (3) In alternative example 3, x:y= 1:4 5x+6y=29 Let x=t and y=4t. Equation 2 can be written as: 5t+6*4t=29 29t=29 t= 1, so x= 1, y. Projection theorem of right triangle (also called Euclid theorem): In a right triangle, the height on the hypotenuse is the proportional average of the projections of two right angles on the hypotenuse. Each right-angled edge is the median of the projection of this right-angled edge on the hypotenuse and the proportion of the hypotenuse. The content of junior high school projective theorem: the content of projective theorem is: in a right triangle, each right-angled side is the median of the projection of this right-angled side on the hypotenuse, and the high line on the hypotenuse is the median of the projection of two right-angled sides on the hypotenuse.