1, multiplication rule: the fraction is multiplied by the fraction, the product of the molecule is the numerator of the product, and the product of the denominator is the denominator of the product.

2. Add and subtract fractions with different denominators, first divide them into fractions with the same denominator, and then add and subtract.

3. In a right triangle, if an acute angle is equal to 30, then the right side it faces is equal to half of the hypotenuse.

If the sum of squares of two sides of a triangle is equal to the square of the third side, then the triangle is a right triangle.

5. Generally, several linear inequalities about the same unknown are combined to form a linear inequality group.