Đáp án:
`D=sqrt5+1`
Giải thích các bước giải:
`D=sqrt(4+sqrt(10+2sqrt5))+sqrt(4-sqrt(10+2sqrt5))`
`D^2=4+sqrt(10+2sqrt5)+4-sqrt(10+2sqrt5)+2sqrt((4+sqrt(10+2sqrt5))(4-sqrt(10+2sqrt5))`
`D^2=8+2sqrt(16-10-2sqrt5)`
`D^2=8+2sqrt(6-2sqrt5)`
`D^2=8+2sqrt(5-2sqrt5+1)`
`D^2=8+2sqrt((sqrt5-1)^2)`
`D^2=8+2(sqrt5-1)=8+2sqrt5-2`
`D^2=6+2sqrt5=5+2sqrt5+1`
`D^2=(sqrt5+1)^2`
`D=sqrt5+1`
