Explicație pas cu pas:
a=2/3*5+3/5*8+4/8*12=>
a=(5-3)/3*5+(8-5)/5*8+(12-8)/8*12=>
a=5/3*5-3/3*5+8/5*8-5/5*8+12/8*12-
-8/8*12 =>
a=1/3-1/5+1/5-1/8+1/8-1/12 =>
a=1/3-1/12 => a=4/12-1/12=>a=3/12 =>
a=1/4
a€(1/10,1/2)<=>
1/10<a<1/2 <=>
1/10<1/4<1/2
Cum 1/10<1/4 (4<10) si 1/4<1/2(2<4) =>
1/10<1/4<1/2 (Adevarat) => a€(1/10,1/2)