`a)`
Ta có :
`3^21 = 3^20 . 3 = (3^2)^10 . 3 = 9^10 . 3`
`2^31 = 2^30 . 2 = (2^3)^10 . 2 = 8^10 . 2`
Vì `8<9 to 8^10<9^10`
Mà `2<3 to 8^10 . 2 < 9^10 . 3`
hay `2^31 < 3^21`
`b)`
`B=sqrt(x-1)+2`
Ta có : `sqrt(x-1) ge 0 \ AA x`
`to sqrt(x-1)+2 ge 2`
Dấu "=" xảy ra khi : `sqrt(x-1)=0`
`<=> x=1`
Vậy `B_(min)=2 <=> x=1`