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Dot-product of a vector with vectors 3i - 5k, 2i + 7j and i + j + k are respectively -1, 6 and 5. Find the vector.

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∴ 3x – 5z = -1 … (1) 

∴ 2x + 7y = 6 … (2) 

∴ x + y + z = 5 … (3) 

From (3), z = 5 – x – y

Substituting this value of z in (1), we get

∴ 3x – 5(5 – x – y)= -1 

∴ 8x + 5y = 24 … (4)

Multiplying (2) by 4 and subtracting from (4), we get

8x + 5y – 4(2x + 7y) = 24 – 6 × 4 

∴ -23y = 0 ∴ y = 0 

Substituting y = 0 in (2), we get

∴ 2x = 6 

∴ x = 3 

Substituting x = 3 in (1), we get 

∴ 3(3) – 5z = -1 

∴ 5z = -10 

∴ z = 2

Hence, the required vector is \(3\hat{i} + 2\hat{k}\)

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