1, perimeter formula: The following types of perimeter formulas are common in junior high schools:
Rectangular perimeter = (length+width) ×2, C=2(a+b)
Square perimeter = side length ×4, C=4a.
Circumference = diameter × π? ，C=2πr .
2, the area formula: junior high school geometric area formula is common in the following categories:
Rectangular area = length × width, S=ab.
Square area = side length × side length, S=a? .
Triangle area = base × height ÷2, S=ah/2 parallelogram area = base × height, S=ah? Trapezoidal area = (upper bottom+lower bottom) × height ÷2, S= 1/2(a+b)h? Circular area = radius× radius× π, S=πr Sector area = radius× radius× π× central angle (n)÷360, S=nπr? /360。
3. Linear function formula: The linear function is a straight line, and the expression is as follows.
Point oblique type: y-b = k (x-a); The slope k and the intersection (a, b) are known.
Two-point formula: (y-b)/(x-a) = (b-d)/(a-c); It is known that the slopes of two points (a, b) and (c, d) are (b-d)/(a-c): y = kx+b; Given the slope k, the y-axis intercept is b, that is, the intersection point (0, b) is inclined according to the point.
Interception formula: x/a+y/b =1; It is known that the intercepts of X-axis and Y-axis are a and b respectively, that is, they pass through two points (a, 0) and (0, b) according to the two-point formula.
4. Quadratic function expression? Quadratic function is a parabola, and there are three expressions.
General formula: y=ax? +bx+c； (a≠0)？
Vertex: y=a(x-h)? +k； [a≠0 fixed point (h, k)]?
Intersection point: y = a (x-x1) (x-x2); [The parabola intersects the X axis at (x 1, 0)(x2, 0)]
5. Quadratic function picture: quadratic function expression y=ax? +bx+c； Quadratic function is an axisymmetric figure.
The quadratic coefficient a determines the opening direction (a > 0, the opening is upward; A < 0, opening down)?
Symmetry axis: x = -b/2a
Vertex coordinates: [-b/2a, (4ac-b? )/4a ]
δ= b？ -4ac；
The number of times the parabola intersects the X axis (δ > 0, it intersects twice; When δ = 0, 1 intersection; When δ < 0, there is no intersection).