I. a) ((8/10)^-2 +(6/10)^-2)^-1 =((10²/8²) +(10²/6²))^-1 =(100/64 +100/36)^-1 =(3600+6400)/2304)^-1 =2304/10000 =0.2304
b) =(5/2² -20/5²) :(3²/4²) =(5/4 -4/5) *16/9 =(25 -16)*16 /20*9 =9*16 /20*9 =16/20 =4/5 =0.8
II. =a^-3(1 -a²) /(-a^-2(1 -a²) = -a²/a^3 = -1/a
III. (x^-1 -y^-1)^-2 =(1/x -1/y)^-2 =((y-x)/xy))^-2 =x²y² /(y-x)²
(x^-2 -y^-2)^-1 =(1/x² -1/y²)^-1 =((y²-x²)/x²y²)^-1 =x²y² /(y²-x²) =x²y²/(y-x)(y+x)
x²y² /(y-x)² * (y-x)(y+x)/x²y² =(x²y² *(y-x)(y+x)) /(y-x)² *x²y² =(y+x)/(y-x)
ответ: (y +x) /(y -x)