Arithmetic · real student question

Evaluate the greatest integer less than or equal to 100000 times 32 divided by (4 ln 2 + 3).

Question

Evaluate

100000324ln2+3\left\lfloor 100000\cdot\frac{32}{4\ln 2+3}\right\rfloor

Step-by-step solution

  1. Interpret the bracket as the floor function. In this notation [][\,\cdot\,] means the greatest integer not exceeding the value, so the task is to compute a quotient and then truncate downward. The whole difficulty is deciding the integer part correctly, which requires knowing how close the quotient sits to an integer.

  2. Collapse the numerator. 100000×32=3,200,000100000\times32=3{,}200{,}000, so we need

    3,200,0004ln2+3\left\lfloor\frac{3{,}200{,}000}{4\ln 2+3}\right\rfloor

  3. Evaluate the denominator to enough precision. With ln2=0.69314718055994531\ln 2=0.69314718055994531,

    4ln2=2.77258872223978124,4ln2+3=5.772588722239781244\ln 2=2.77258872223978124,\qquad 4\ln2+3=5.77258872223978124

    Truncating ln2\ln2 to three decimals would shift the quotient by about ±200\pm 200, which is far too coarse for a floor — this is why ten or more digits are needed.

  4. Divide.

    3,200,0005.77258872223978124=554,344.0134010431\frac{3{,}200{,}000}{5.77258872223978124}=554{,}344.0134010431

  5. Take the floor and confirm the margin. The fractional part is 0.01340.0134, so the floor is 554,344554{,}344. The result is genuinely close to the boundary: multiplying back, 554,344×5.77258872224=3,199,999.923554{,}344\times5.77258872224=3{,}199{,}999.923, which is just under 3,200,0003{,}200{,}000, while 554,345×5.77258872224=3,200,005.70554{,}345\times5.77258872224=3{,}200{,}005.70 overshoots ✓. So 554,344554{,}344 is correct and 554,345554{,}345 is not.

Answer

554344554344

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