\(\left(xy-1\right)^5=C_5^0.\left(xy\right)^5+C^1_5.\left(xy\right)^4.\left(-1\right)+C^2_5.\left(xy\right)^3.\left(-1\right)^2+C^3_5\left(xy\right)^2.\left(-1\right)^3+C^4_5.xy.\left(-1\right)^4+C^5_5.\left(-1\right)^5\)
\(=\left(xy\right)^5-5\left(xy\right)^4+10\left(xy\right)^3-10\left(xy\right)^2+5xy-1\)