When the bending moment M$=50$kNm acts on a reinforced concrete beam section with a width b$=300$mm and a depth of $600$mm $($effective depth d$=500$mm$,$ reinforced cross$-$sectional area As$=2$x$200=400$mm$^2)1.$ Find the distance from the compression podium to the neutral axis and the distance from the tensile podium to the neutral axis$($Hint: Concrete elastic modulus Ec$=25,000$ Mpa, elastic modulus Es$=200,000$ Mpa of rebar, elastic modulus ratio n$=$Es$/$Ec$=8$ of rebar equivalent$)$It can be replaced by concrete area $($nAs$)2.$ Find the second moment of the cross section with respect to the neutral axis$($Hint: When using the parallel axis theorem, the secondary moment of the reinforcement's own cross section can be ignored.$)3.$ Find the vertical stress of the compression end and the vertical stress of the tensile end caused by the bending moment$4.$ When the tensile strength of concrete is $2$ Mpa, determine if cracks occur in the beam section