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(D) According to Newton's lens formula,for a convex lens,the product of the distances of the object and the image from the respective foci is equal to

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$x + x' \ge 2f$

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(D) According to Newton's lens formula,for a convex lens,the product of the distances of the object and the image from the respective foci is equal to the square of the focal length,i.e.,$xx' = f^2$.For a real image to be formed by a convex lens,the object must be placed beyond the focus $(x > 0)$.By the Arithmetic Mean-Geometric Mean ($AM$-$GM$) inequality,for positive real numbers $x$ and $x'$,we have $\frac{x + x'}{2} \ge \sqrt{xx'}$.Substituting $xx' = f^2$,we get $\frac{x + x'}{2} \ge \sqrt{f^2} = f$.Therefore,$x + x' \ge 2f$.

An object is placed at a point distant $x$ from the focus of a convex lens and its image is formed at $I$ as shown in the figure. The distances $x$ and $x'$ satisfy the relation:

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