f(2x-8)=x(kuadrat)-4x,maka nilai dari f(6) adalah

Diketahui :
f(2x-8) = x² - 4x

Ditanya :
f(6) =???

Dijawab :

Cara 1 :
Misal :
2x - 8 = m
2x = m + 8
x = ½(m+8)

[tex]f(x) = ( \frac{m + 8}{2} ) {}^{2} - 4( \frac{m + 8}{2} ) \\ \\ = \frac{ {m}^{2} + 16m + 64 }{4} - 2m - 16 \\ \\ = \frac{ m^{2} + 16m + 64 - 8m - 64 }{4} \\ \\ = \frac{ {m}^{2} + 8m}{4} \\ \\ f(6) = \frac{ {6}^{2} + 8(6)}{4} = \frac{36 + 48}{4} \\ \\ = \frac{84}{4} \\ \\ = 21[/tex]

Cara 2
f(2x-8) = x² - 4x
f(6) = ???

2x - 8 = 6
2x = 14
x = 7

f(6) = 7² - 4(7)
f(6) = 49 - 28
f(6) = 21