Đáp án:
`B=2/(sqrt7-5)-2/(sqrt7+5)`
`B=(2(sqrt7+5))/(7-25)-(2(sqrt7-5))/(7-25)`
`B=(2(sqrt7+5)-2(sqrt7-5))/(-18)`
`B=20/(-18)`
`B=-10/9`
`C=(sqrt7+sqrt5)/(sqrt7-sqrt5)+(sqrt7-sqrt5)/(sqrt7+sqrt5)`
`C=(sqrt7+sqrt5)^2/(7-5)+(sqrt7-sqrt5)^2/(7-5)`
`C=(7+2sqrt{7.5}+5+7-2sqrt{7.5}+5)/2`
`C=24/2=12`
`D=1/(sqrt2+1)+1/(sqrt3+sqrt2)+1/(sqrt4+sqrt3)`
`D=(sqrt2-1)/(2-1)+(sqrt3-sqrt2)/(2-1)+(sqrt4-sqrt3)/(4-3)`
`D=sqrt2-1+sqrt3-1+sqrt4-sqrt3`
`D=sqrt4-1`
`D=2-1=1`