N
ngoc2291998


cho hinh thang ABCD(AB song song voi CD).O la giao diem 2 duong cheo.Qua O ke duong thang song song voi 2 day cat BC tai I;AD tai J. C/m:
a) 1/OI=1/AB+1/CD
b)2/IJ=1/AB+1/CD
a) 1/OI=1/AB+1/CD
b)2/IJ=1/AB+1/CD