角C=90°,BC=3,cosA=根号3/ 2
AB=AC/cosA
AC^2+BC^2=AB^2
AC^2+3^2=(AC/cosA)^2=[AC/(根号3/2)]^2=4AC^2/3
4AC^2/3-AC^2=3^2
AC^2=3*3^2
ac=3根号3