d)
Ta có:
`1/a+1/b+1/c=2`
`⇒(1/a+1/b+1/c)^2=4`
`⇒1/a^2+1/b^2+1/c^2+2.(1/(ab)+1/(bc)+1/(ca))=4`
`⇒1/a^2+1/b^2+1/c^2=4-2.(1/(ab)+1/(bc)+1/(ca))`
`⇒1/a^2+1/b^2+1/c^2=4-2.(c/(abc)+a/(abc)+b/(abc))`
`⇒1/a^2+1/b^2+1/c^2=4-2.(a+b+c)/(abc)`
`⇒1/a^2+1/b^2+1/c^2=4-2.(abc)/(abc)`
`⇒1/a^2+1/b^2+1/c^2=4-2`
`⇒1/a^2+1/b^2+1/c^2=2`
