in tri. ADC,sin 30=DC/17
in tri. BAD,sin 45=BD/17
when DC+BD=a
so a=17sin45+17sin30
=17(sqrt3)/2+17(sqrt2)/2
=26.743...
=26.7,cor to 3 sig fig
2011-09-29 09:36:19 補充:
error
in tri. ADC,sin 30=DC/17=1/2
in tri. BAD,sin 45=BD/17=sqrt2/2
when DC+BD=a
so a=17sin45+17sin30
=17(sqrt3)/2+17(0.5)
=23.2224319
=23.2 cor to 3 sig fig