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Fie ABCD romb
P=4l=>l=P/4=68/4=17 cm
AC=30 cm
AC∩BD={O}
AO=OC=AC/2=30/2=15 cm
In ΔAOB , m(∡O)=90°=>BO²=AB²-AO²=17²-15²=289-225=64=>BO=√64=8 cm
BO=OD=8 cm
BD=BO+OD=8+8=16 cm