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In a parallelogram ABCD. Prove that vec(...

In a parallelogram ABCD. Prove that `vec(AC)+ vec (BD) = 2 vec(BC)`

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`vec (AC) = vec(AB) +vec (BC) & vec (BD) + vec (CD)` [applying triangle law of vectors]
Now `vec(AC) + vec(BD) = vec(AB) + vec (BC) + vec(BC) + vec(CD) = vec(AB) + 2vec(BC) + vec(CD)`
But `vec(CD) =-vec(AB)` `" "therefore vec(AC) + vec(BD) = vec(AB) + 2 vec(BC) - vec(AB) = 2vec(BC)`
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