What is that least number that must be added to the product 684 × 686 to make it a perfect square?
Number = $$684\times686$$
= $$684\times(684+2)$$
= $$(684)^2+2(684)$$
Now, to make it a perfect square, = $$(684)^2+2(684)(1)+(1)^2$$
= $$(684+1)^2=(685)^2$$
$$\therefore$$ 1 is added to make it a perfect square.
=> Ans - (B)
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