Sum of four-month principal and interest =100+4 *100 * 0.06% =100.24.

2. When the side length increases X, the area increases Y,: Y = (3+x) 2-3 * 3.

Resolution function of x and y: (3+x) 2 = y-9x = [under (y-9) root sign ]-3 (y > =9) independent variable is y.

The function is x=[ (y-9) under the root sign ]-3.

When x is equal to 1, 2, 3 and 4, the values of y are 7 16 27 40 respectively.

3:y = 500+20x-25x =-5x+500(0 = & lt； x & lt= 100)