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Each standard deviation represents a fixed percentile. Thus, rounding to two decimal places, −3σ is the 0.13th percentile, −2σ the 2.28th percentile, −1σ the 15.87th percentile, 0σ the 50th percentile (both the mean and median of the distribution), +1σ the 84.13th percentile, +2σ the 97.72nd percentile, and +3σ the 99
If a data distribution is approximately normal then about 68 percent of the data values are within one standard deviation of the mean (mathematically, μ ± σ, where μ is the arithmetic mean), about 95 percent are within two standard deviations (μ ± 2σ), and about 99.7 percent lie within three standard deviations (μ ± 3σ).
With a correct value for its seven first decimal digits, this value remained the most accurate approximation of π available for the next 800 years. [54] The Indian astronomer Aryabhata used a value of 3.1416 in his Āryabhaṭīya (499 AD). [55] Fibonacci in c. 1220 computed 3.1418 using a polygonal method, independent of Archimedes. [56]
In general, if an increase of x percent is followed by a decrease of x percent, and the initial amount was p, the final amount is p (1 + 0.01 x)(1 − 0.01 x) = p (1 − (0.01 x) 2); hence the net change is an overall decrease by x percent of x percent (the square of the original percent change when expressed as a decimal number).
Zu Chongzhi is known to have computed π to be between 3.1415926 and 3.1415927, which was correct to seven decimal places. He also gave two other approximations of π : π ≈ 22 ⁄ 7 and π ≈ 355 ⁄ 113 , which are not as accurate as his decimal result.
For example, in duodecimal, 1 / 2 = 0.6, 1 / 3 = 0.4, 1 / 4 = 0.3 and 1 / 6 = 0.2 all terminate; 1 / 5 = 0. 2497 repeats with period length 4, in contrast with the equivalent decimal expansion of 0.2; 1 / 7 = 0. 186A35 has period 6 in duodecimal, just as it does in decimal. If b is an integer base ...
An example of an irrational algebraic number is x 0 = (2 1/2 + 1) 1/3. It is clearly algebraic since it is the root of an integer polynomial, (x 3 − 1) 2 = 2, which is equivalent to x 6 − 2x 3 − 1 = 0. This polynomial has no rational roots, since the rational root theorem shows that the only possibilities are ±1, but x 0 is
In decimal numbers greater than 1 (such as 3.75), the fractional part of the number is expressed by the digits to the right of the decimal (with a value of 0.75 in this case). 3.75 can be written either as an improper fraction, 375/100, or as a mixed number, 3 + 75 / 100 .