`a)` Sai

`y>=0`

`=> x^2+4x-5>=0`

`=> (x-1)(x+5)>=0`

`=> x>=1` hoặc `x<= -5`

`-> x in [1; +infty) uu [-infty;-5]`

`b)` Sai 

`y<=0`

`=> x^2+4x-5<=0`

`=> (x-1)(x+5)<=0`

`=> -5<=x<=1`

`-> x in [-5;1]`

`c)` Đúng

`x^2+4x-5=x^2+4x+4-9=(x+2)^2-9>=0-9=-9`

`=> min=-9 <=> x=-2`.

`d)` Đúng

Xét PTHĐGĐ

`x^2+4x-5=4x-5/2`

`<=> x^2+4x-5-4x+5/2=0`

`<=> x^2-5/2=0`

`<=> x^2=5/2`

`<=> x_1=[sqrt10]/2` và `x_2=-[sqrt10]/2`

Đề:

`x_1^2+x_2^2=5`

`<=> 5/2+5/2=5`

`<=> 5=5` (đúng)