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This is suppossed to be square of the sum of two terms if we complete the square. In such a perfect square, we will find the squares of both the terms and twice the product of the terms. Here in your problem only one square of a term i.e.

is found and another is not found. So 'k' should be another square. Because 10x is having the first term(x) and 10 should contain the

and 2 as factors. So 10 when divided by 2, we get 5. So 5 is another term. But to complete the square, we take the square of 5 in the place of 'k'. i.e. k=25.