`a)m=0`
`pt<=>x^2-2.(0+2)x+0+0+3=0`
`<=>x^2-4x+3=0`
`Delta'=4-3=1`
`=>x_1=1,x_2=3`
Vậy `m=0` thì pt có `S={1,3}`
b)PT có 2 nghiệm
`<=>Delta'>=0`
`<=>(m+2)^2-m^2-m-3>=0`
`<=>m^2+4m+4-m^2-m-3>=0`
`<=>3m+1>=0`
`<=>m>=-1/3`
Áp dụng vi-ét ta có:
`x_1+x_2=2(m+2),x_1.x_2=m^2+m+3`
`x_1/x_2+x_2/x_1=4`
`<=>(x_1^2+x_2^2)/(x_1.x_2)=4`
`<=>((x_1+x_2)^2-2x_1.x_2)/(x_1.x_2)=4`
`<=>(x_1+x_2)^2/(x_1.x_2)-2=4`
`<=>(4(m+2)^2)/(m^2+m+3)=4+2=6`
`<=>(2(m+2)^2)/(m^2+m+3)=3`
`<=>2(m^2+4m+4)=3m^2+3m+9`
`<=>m^2-5m+1=0`
`Delta=25-4=21`
`=>m_1=(sqrt{21}+5)/2(tm),m_2(-sqrt{21}+5)/2(tm)`
Vậy `m={(sqrt{21}+5)/2,(-sqrt{21}+5)/2}`
