apabila f(x)=9x- x^{2} , berapakah nilai f(2)-f(2 \sqrt{5} =.....

[tex]f(x) = 9x - x^{2}[/tex]
f(2) = (9 x 2) - [tex]2^{2}[/tex] = 18 - 4 = 14
f(2[tex]\sqrt{5}[/tex]) = (9 x 2[tex]\sqrt{5}[/tex]) - [tex](2\sqrt{5})^{2}[/tex] = 18[tex]\sqrt{5}[/tex] - 20
f(2) - f(2[tex]\sqrt{5}[/tex]) = 14 - (18[tex]\sqrt{5}[/tex] - 20) = 14 - 18[tex]\sqrt{5}[/tex] + 20 = 34 - 18[tex]\sqrt{5}[/tex]