1, let the long right angle side of a right triangle be X, then the other one is X-5. Because the area is 7, solving X(X-5)/2=7 results in X=7, so the other right-angled side is 2. According to Pythagorean theorem, the hypotenuse is at the root sign (7? +2? ) = 53 under the root sign;

2. Suppose the average annual growth rate is x%, then there is 1200 (1+x)? = 1452，x = 0. 1 = 10%；

3, this is 10 companies, I think so, set up X companies, so you can think of the combination of two from X.

X(X- 1)/2=45, so we get X= 10. I don't know what level 9 is now. As far as I can remember, the arrangement combination was only learned in high school. Under the calculation, it is assumed that there are 10 units. Draw a unit first, sign the rest, and then leave. This is nine times, and the rest is left.