|....||......||......|| ____|..........||.........||................| ______|....................||...........................|..
O A₁ A₂ A₃ A₄₉ M A₅₀ A₅₁ A₉₈ A₉₉ A₁₀₀
A₄₉M=MA₅₀ =A₄₉A₅₀/2= 50/2=25
A₄₀M=40+41+42+....48+49+25 =(40+49)*10/2 +25=
=89* 5 +25 = 445+25= 470