问题一:求证,B>=π/3
问题二:求证,a+c<=2b
问题一:求证,B>=π/3
问题二:求证,a+c<=2b
A+C<=2B
A+B+C<=3B
π<=3B
即B>=π/3
第二个我再想想
1.3B=B+2B≥B+(A+C)=π,故B≥π/3
2.(a+c)/b=(sinA+sinC)/sinB
=2sin[(A+C)/2]cos[(A-C)/2]/sinB
=2cos(B/2)cos[(A-C)/2]/sinB
=cos[(A-C)/2]/sin(B/2)
而B/2∈[π/6,π/2],sin(B/2)≥1/2
cos[(A-C)/2]∈(0,1]
故cos[(A-C)/2]/sin(B/2)∈(0,2]
(a+c)/b∈(0,2]
a+c≤2b