Jika 3 pangkat x-2y=1/81 dan 2 pangkat x-y=16. Maka nilai x+y adalah..

3^(x - 2y) = 1/81 ................(1)
2^(x - y) = 16 ......................(2)

(1)
3^(x - 2y) = 1/81
3^(x - 2y) = 1/(3⁴)
3^(x - 2y) = 3⁻⁴
x - 2y = -4 .................(i)

(2)
2^(x - y) = 16
2^(x - y) = 2⁴
x - y = 4 ..................(ii)

dari (i) dan (ii)
x - 2y = -4
x - y = 4
------------- (-)
y = 8

subtitusi
x - y = 4
x - 8 = 4
x = 12

jadi x + y = 12 + 8 = 20