Răspuns:
n=n1n2(n1+n2-n1n2)
n=36/49
n=73.4%
Explicație:
in primul caz: n1=R/(R+r1)
R=n1(R+r1)
r1=R(1-n1)/n1
in cazul sl doilea: r2=R(1-n2)/n2
in serie:
n=R/(R+r1+r2)=1/[1+(1-n1)/n1+(1-n2)/n2].
n=1/(1+1/n1-1+1/n2-1)=1/(1/n1+1/n2-1)
n=n1n2(n1+n2-n1n2)
Numeric:
n=0.9×0.8/(0.9+0.8-0.72)
n=0.72/0.98=72/98= 36/49=0.734
n=73.4%