A rectangular single$-$reinforced beam with a cross$-$section as shown in the figure $1,$ and its tensile steel bars are $6-$ D$19.$ The concrete strength is $280kg\frac{f}{c}{m}^2,$ and the steel bar strength is $2800kg\frac{f}{c}{m}^2.$

$($a$)$When the beam is subjected to a positive bending moment of $8tf-m,$ try to calculate the stress of the steel bar and the maximum compressive stress of the concrete. $(10\%)$

$($b$)$Try using the equivalent rectangular stress distribution method to find the design strength of the bending moment of the reinforced concrete beam. $(15\%)$

$($Hint : $M_{cr}=2f_r*\frac{I_g}{h})$