a寸是（长）*（宽）cm5寸是12.7*8.9cm6寸是15.2*10.2cm7寸是17.8*12.7cm8寸是20.3*15.2cm10寸是25.4*20.3cm12寸是30.4*25.4cm16寸是40.0*30.0cm18寸是45.0*35.0cm