For example: (1) 2x-3y? -7x-5y？ =(2-7)x+(-3-5)y？ =-5x+(-8)y？ =-5x-8y？ .
(2) 5a？ +2a-4a？ +7a-3a？ =5a？ +(2+7)a+(-4-3)a？ =5a？ +9a-7a？ .
2. Algebraic letters are usually preceded by a "+or-"symbol, which can be regarded as "+"when not written.
Rules for removing brackets:
(1) When there is a "+"before the bracket, all items in the bracket do not need to be changed after removing the bracket and the "+"before it;
Such as: (-a+b)+(3c-8) =-a+b+3c-8;
Another example is: (x+2y)+(-3z+m) = x+2y-3z+m.
(2) When there is a "-"in front of the bracket, after removing the bracket and the "-"in front, each item in the bracket should be changed.
Such as:-(a+b)-(-c-3) =-a-b+c+3;
Another example is: -(-x+y)-(z-4)=x-y-z+4.
3. When to use fractions in algebraic expressions is of course determined according to the content of the topic.
For example, the sum of twice x and half y can be expressed as 2x+(1/2) y;
For another example, if the upper bottom of a trapezoid is A, the lower bottom is B and the height is H, then its area is (1/2) (A+B) H.