Kẻ MN vuông góc vs CI;MP vuông góc vs AB
CI:2x+y-10=0$\Rightarrow$ MN:x-2y+5=0
N=CI$\cap$MN$\Rightarrow$N(3;4)
MP:2x+y-$\frac{25}{2}$=0
P=MP$\cap$AB$\Rightarrow$P(5;$\frac{5}{2}$)
S MNIP=MN.MP=$\frac{5}{2}$
Gsử$\frac{BP}{BI}$=k$\Rightarrow$$\frac{S MBP}{S CIB}$=$k^{2}$
$\frac{S CMN}{S CIB}=(1-k)^{2}$
S ABC=10$\Rightarrow$ S CIB=5
Mà $S MNIP=S CIB=S MBP- S CMN$$\Rightarrow$$S CIB.\left[ {1-(1-k)^{2}-k^{2}} \right]=\frac{5}{2}$
$\Rightarrow$k=$\frac{1}{2}$
$\Rightarrow$$2\overrightarrow{BP}=\overrightarrow{BI} \Rightarrow B(6;3)$
Đến đây bn tự tìm nốt tọa độ A;C