Given f(x,y)=80x+60y4x25y2xy\displaystyle {f{{\left({x},{y}\right)}}}={80}{x}+{60}{y}-{4}{x}^{{2}}-{5}{y}^{{2}}-{x}{y}

Evaluate f(5,8)\displaystyle {f{{\left({5},{8}\right)}}}

f(5,8)\displaystyle {f{{\left({5},{8}\right)}}} =