What focal length should the reading spectacles have for a person for whom the least distance of vision is 50 cm ? (The distance of Normal vision is 25 cm)

The sum of all coefficients of&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mo>(</mo><mn>2</mn><mi mathvariant="normal">x</mi><mo>&nbsp;</mo><mo>+</mo><mo>&nbsp;</mo><mn>3</mn><mi mathvariant="normal">y</mi><mo>&nbsp;</mo><mo>–</mo><mo>&nbsp;</mo><mn>7</mn><mi mathvariant="normal">z</mi><mo>)</mo></mrow><mi mathvariant="normal">n</mi></msup></math>&nbsp;is 1024. Then n =

The coefficient of xn in the expansion of&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mo>(</mo><mn>1</mn><mo>+</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>3</mn><msup><mi mathvariant="normal">x</mi><mn>2</mn></msup><mo>+</mo><mo>&nbsp;</mo><mo>.</mo><mo>.</mo><mo>.</mo><mo>.</mo><mo>.</mo><mo>)</mo></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup></math>&nbsp;is&nbsp; &nbsp;

<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><mn>1</mn><mo>+</mo><mfrac><msub><mi mathvariant="normal">C</mi><mn>1</mn></msub><msub><mi mathvariant="normal">C</mi><mn>0</mn></msub></mfrac></mrow></mfenced><mfenced><mrow><mn>1</mn><mo>+</mo><mfrac><msub><mi mathvariant="normal">C</mi><mn>2</mn></msub><msub><mi mathvariant="normal">C</mi><mn>1</mn></msub></mfrac></mrow></mfenced><mfenced><mrow><mn>1</mn><mo>+</mo><mfrac><msub><mi mathvariant="normal">C</mi><mn>3</mn></msub><msub><mi mathvariant="normal">C</mi><mn>2</mn></msub></mfrac></mrow></mfenced><mo>…</mo><mo>…</mo><mfenced><mrow><mn>1</mn><mo>+</mo><mfrac><msub><mi mathvariant="normal">C</mi><mi mathvariant="normal">n</mi></msub><msub><mi mathvariant="normal">C</mi><mrow><mi mathvariant="normal">n</mi><mo>-</mo><mn>1</mn></mrow></msub></mfrac></mrow></mfenced></math>&nbsp;is equal to&nbsp; &nbsp;

The numerically greatest term in the expansion of&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mo>(</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo>&nbsp;</mo><mo>–</mo><mo>&nbsp;</mo><mn>6</mn><mi mathvariant="normal">y</mi><mo>)</mo></mrow><mn>14</mn></msup></math>, when x = 2/5, y = 1/2 is

What focal length should the reading spectacles have for a person for whom the least distance of vision is 50 cm ? (The distance of Normal vision is 25 cm)

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