Ban tu ve hinh ra, chu y them duong ke phu AM
Theo de bai \(M\in BM\)sao cho \(BM=MC\)va \(N\in AC\)sao cho \(AN=NC\)
suy ra \(\Delta AMC=\frac{1}{2}\Delta ABC\)(vi co chung chieu cao ha tu A xuong BC, BM bang 1/2 BC)
Xet tam giac AMC co \(\Delta AMN=\frac{1}{2}\Delta AMC\)(vi co chung chieu cao ha tu M xuong AB , AN bang 1/2 AC)
Tu 2 dieu tren ta co \(\Delta AMN=\frac{1}{2}\times\frac{1}{2}=\frac{1}{4}\Delta ABC\)
P/S con thieu gi thi mong b thong cam, mk lau chua dung vao bai nhu v