7
1)1/(a+5)-1/(a-5)=(a-5-a-5)/(a²-25)=-10/(a²-25)
2)a(a²-25)*[-10/(a²-25)]=-10a
a=-39 -10*(-39)=390
8
1)1/5a+1/7a=(7+5)/35a=12/35a
2)12/35a * a²/4=3a/35
a=7,7 3*7,7/35=3*1,1/5=3,3/5=0,66
9
1)y/5x-5x/y=(y²-25x²)/5xy
2)(y-5x)(y+5x)/5xy *1/(y+5x)=(y-5x)/5xy
x=1/7;y=1/4
(1/4-5/7):(5*1/4*1/7)=-13/28*28/5=-13/5=-2,6
10
(x-3)*(x+3)/(x-3)²=(x+3)/(x-3)
x=-21
(-21+3)/(-21-3)=-18/(-24)=3/4=0,75
11
16/a(4-a)-4/a=(16-16+4a)/a(4-a)=4/(4-a)
a=-12
4/(4+12)=4/16=1/4=0,25