155)2с*с/а^2-c^2 *a^2+ac/c^2=2с* с/(а-c)(a+c)*a(a+c)/c^2=2a/(a-c)
156) bc*(b-c)(b+c)/3c *(b-c)/C^2=3bc/(b+c)
157) (x-y)(x+y)/xy * 3y/(x-y) * 1/(x+y)= 1/x
158) a/(a-c)(a+c) *(a+c)/2ac *(a-c)/1=1/2c
159) (y+xy-x+y)/xy=2y+xy-x/xy=(2y-x)/xy
160)(ab-b^2+a^2-a^2-b^2)/ab=-(2b^2)/ab