Giai giup minh chỗ BTVN với cam on nhiu

Giai giup minh chỗ BTVN với cam on nhiu
giai-giup-minh-cho-btvn-voi-cam-on-nhiu

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  1. Bài 3:

    Ta có:

    `vec{AD} + vec{BE} + vec{CF}`

    `= vec{AE} + vec{ED} + vec{BF} + vec{FE} + vec{CD} + vec{DF}`

    `= vec{AE} + vec{BF} + vec{CD} + (vec{ED} + vec{FE} + vec{DF})`

    `= vec{AE} + vec{BF} + vec{CD} + (vec{FD} + vec{DF})`

    `= vec{AE} + vec{BF} + vec{CD}`

    Bài 4:

    `a) vec{DA} – vec{DB} + vec{DC}`

    `= vec{DA} + vec{BD} + vec{DC}`

    `= vec{DA} + vec{BC}`

    `= vec{0}`

    `b) vec{CO} – vec{OB}`

    `= vec{CO} + vec{BO}`

    `= vec{OA} + vec{BO}`

    `= vec{BA}`

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