помогите пожалуйста с 17 18 19 номерами

Решение на фото

17
1)(x+y)/(x+y):(x-y)=(x+y)(x-xy+y)/(x+y) *1/(x-y)=(x-xy+y)/(x-y)
2)(x-xy+y)/(x-y)(x+y)+2y/(x+y)=(x-xy+y+2xy-2y)/(x+y)(x-y)=
=(x+xy-y)/(x-y)(x+y)
3)(x+xy-y)/(x-y)-xy/(x-y)=(x+xy-y-xy)/(x-y)=(x-y)/(x-y)=1
18
1)1/a+1/b=(b+a)ab
2)1/(a+b)*(b+c)/ab=(b+c)/(a+b)ab
3)1/a+1/b=(b+a)/ab
4)2/(a+b)+(b+a)/ab=2/(a+b)ab
5)(b+a)/(a+b)ab+2/(a+b)ab=(b+a+2ab)/(a+b)ab=(a+b)/(a+b)ab=1/ab
6)1/ab*ab=1
19
1)[(x+y)/x+(x-y)/y]=[(xy+y+x-xy)/xy]=(x+y)/xy
2)[(x+y)/x-(x-y)/y]=[(xy+y-x+xy)/xy]=(y-x)/xy
3)(x+y)/xy-(y-x)/xy=(x+y-y+x)(x+y+y-x)/xy=
=2x*2y/xy=4