f(x+2) * f(x) = 1

f(x+4) * f(x+2) = 1

∴ f(x+4) = f(x), that is, the period of f(x) is 4.

f(x+2) * f(x) = 1

Let x=- 1

f( 1)*f(- 1) = 1

F(x) is an even number function, ∴f(- 1)=f( 1).

And f (x) >; 0

∴f(- 1)= f( 1)= 1

f( 1 19)= f(30 * 4- 1)= f(- 1)= 1