Lungumea segmentuluio AB unde A(1;1) si B(3;1) e dat de formula:
AB=[tex] \sqrt{( x_{B}- x_{A})^2+ (Y_{B}- y_{A})^2 [/tex]=[tex] \sqrt{(3-1)^2+(1-1)^2}=2,AC= \sqrt{(3-1)^2+(3-1)^2}=2 \sqrt{2}, [/tex] BC=[tex] \sqrt{(3-3)^2+(3-1)^2}=2 [/tex]. Deci AB=BC, avem triunghi isoscel.
