Jika 4^x + 4^-x = 7, maka nilai 8^x + 8^-x = ....

Misal
a = 2^x
b = 2^-x
ab = 2^x . 2^-x = 2^0 = 1

4^x + 4^-x = 7
2^2x + 2^-2x = 7
(2^x)^2 + (2^-x)^2 = 7
a^2 + b^2 = 7
(a + b)^2 - 2ab = 7
(a + b)^2 - 2(1) = 7
(a + b)^2 = 9
(a + b) = 3

8^x + 8^-x = 2^3x + 2^-3x
= (2^x)^3 + (2^-x)^3
= a^3 + b^3
= (a + b)^3 - 3ab(a + b)
= (3)^3 - 3(1)(3)
= 27 - 9
= 18