Đáp án + Giải thích các bước giải:
`a, \ 1/2 xx 2/3 + 3/4 xx 1/2 + 1/3 xx 1/2 + 1/2 xx 1/4`
`= 1/2 xx ( 2/3 + 3/4 + 1/3 + 1/4 )`
`= 1/2 xx [ ( 2/3 + 1/3 ) + ( 3/4 + 1/4 ) ]`
`= 1/2 xx ( 3/3 + 4/4 )`
`= 1/2 xx ( 1 + 1 ) `
`= 1/2 xx 2 = 1`
`b, \ (1+2+3+...+1001) xx ( 7/2 - 3 1/2 )`
`= (1+2+3+...+1001) xx ( 7/2 - 7/2 )`
`= (1+2+3+...+1001) xx 0`
`= 0`
