What is the final product formed when copper sulphate solution reacts with excess $KCN$?

Complexes of $CN^-$ ligand are generally...

Which of the following properties does the $CN^-$ ion possess: $(i)$ reducing agent,$(ii)$ oxidizing agent,and $(iii)$ complexing agent?

The formula of the product formed, when sodium thiosulphate solution is added to silver bromide is

$FeSO_4$ solution mixed with $(NH_4)_2SO_4$ solution in $1:1$ molar ratio gives the test of $Fe^{2+}$ ion,but $CuSO_4$ solution mixed with aqueous ammonia in $1:4$ molar ratio does not give the test of $Cu^{2+}$ ion. Explain why?

$Ag^{+} + NH_3 \rightleftharpoons [Ag(NH_3)]^{+}$; $k_1 = 3.5 \times 10^{-3}$
$[Ag(NH_3)]^{+} + NH_3 \rightleftharpoons [Ag(NH_3)_2]^{+}$; $k_2 = 1.7 \times 10^{-3}$
Then the formation constant of $[Ag(NH_3)_2]^{+}$ is