∵A+C=180°-60°=120°
∴(A+C)/2=60°
∵A=120°-C
∴(A-C)/2
=(120°-2C)/2
=60°-C
∴sinA+sinC
=2sin((A+C)/2)cos((A-C)/2)
=2sin60°cos(60°-C)
=√3cos(60°-C)
∵0°<C<120°
∴-60°<(60°-C)<60°
∴1/2<cos(60°-C)<1
√3/2<√3cos(60°-C)<√3
即,
∵A+C=180°-60°=120°
∴(A+C)/2=60°
∵A=120°-C
∴(A-C)/2
=(120°-2C)/2
=60°-C
∴sinA+sinC
=2sin((A+C)/2)cos((A-C)/2)
=2sin60°cos(60°-C)
=√3cos(60°-C)
∵0°<C<120°
∴-60°<(60°-C)<60°
∴1/2<cos(60°-C)<1
√3/2<√3cos(60°-C)<√3
即,
√3/2<(sinA+sinC)<√3