Jawaban:

Segitiga ADE dan segitiga ABC sebangun, sehingga sisi-sisi yang bersesuaian sebanding.

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell BC over DE end cell equals cell AB over AD end cell row cell BC over DE end cell equals cell fraction numerator AD plus BD over denominator AD end fraction end cell row cell BC over 8 end cell equals cell fraction numerator 4 plus 5 over denominator 4 end fraction end cell row cell BC over 8 end cell equals cell 9 over 4 end cell row BC equals cell 9 over 4 cross times 8 end cell row BC equals cell 18 space cm end cell end table end style

Maka panjang BC adalah begin mathsize 14px style 18 space cm end style