The first question: both sides of the equation are multiplied by 7 at the same time, and the answer is D.42；;
Question 2: Suppose my brother's age is X and his brother's age is 2X this year.
Nine years ago, my brother's age was (X-9) and my brother's age was (2X-9).
X-9=(2X-9) divided by 5.
Multiply both sides of the equation by 5 at the same time and you get
5X-45=2X-9
Move items, 5X-2X=45-9
Merge similar items, 3X=36.
X= 12
That is to say, my brother is 12 years old this year, so I choose C.
Question 3: If a > is known; 0, b<0, and | a | < |b|, simplified | a+b |-a-b |+|-a-b |-b-a |
a+b & lt； 0，a-b & gt； 0，-a-b & gt； 0，b-a & lt； 0.
Original formula =-a-b-(a-b)+(-a-b)+(b-a)
=-a-b-a+b-a-b+b-a
=-4a
The fourth question:
1+2+1= 4 (square of 2)1+2+3+2+1= 9 (square of 3)
1+2+3+4+3+2+1=16 (square of 4)1+2+3+4+5+4+3+2+1= 25 (.
So, the sum is equal to the square of the middle number.
1+2+3+...+100+99+...+1= (10000)100 square.
1+2+3+...+n+(n-1)+...+1= (the square of n)
I hope I can understand. I answered so hard, give me some reward. Hey hey.