17, let an=a 1.

(n- 1)*d，a 1

Aortic second sound

A3= 12 is 3a 1.

3d= 12

D=2, so an=2n and sn = n 2.

ordinary

18, let x 1, x2 belongs to (0,

∞) and x1< x2, f (x1)-f (x2) =1(x12)-1(x2 2) = (x2 2-).

x2^2-x 1^2>； 0, so f(x 1)-f(x2)0.

So this is a subtraction function.

20. let the growth rate be x and the total amount be d.

D*( 1

X)^4=2D

Suppose it takes n years

D*( 1

x)^n=5D

N=0.25 times 2 is the logarithm with base 5.