=(a²-1)(a²+1) /( (a³+a²)+(a+1))= (a-1)(a+1)(a²+1) / (a+1)(a²+1) =a-1.
3^(2x-y)=81
3^(2x-y)=3^4
2x-y=4
x²-y²=5
y=2x-4
x²-(2x-4)²=5
x²-4x²+16x-16=5
3x²-16x+21=0
D=256-252=4
x1=3
x2=7/3
y1=2x-4 =2
y2=2*7/3 - 4= 2/3
D=1+4*20=81^2 D=9