a, `P=(1/{m-m²}+1/{m-1}):{m+1}/{m²-2m+1}`
`=(1/{m(1-m)}-1/{1-m}):{m+1}/(m-1)²`
`=(1/{m(1-m)}-m/{m(1-m)}).(m-1)²/{m+1}`
`={1-m}/{m(1-m)}.(m-1)²/{m+1}`
`=(m-1)²/{m(m+1)}`
b, Thay `m=1/2(tm)` vào P ta được:
`P=(1/2-1)²:(1/2.(1/2+1)`
`=(-1/2)²:(1/{2} .{3}/2)`
`=1/4:3/4`
`=1/3`
Vậy `P=1/3` khi `m=1/2`