## 7. Exercise Set 2

#### Exercise 2.1: Three Circles

Given: an array of three real values, `r`, for the radii of three circles;

Compose: a let-in statement that returns two arrays containing the areas and circumferences of the circles of the given radii.

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#### Exercise 2.2: More Statistics

Given: four three-element arrays, a, b, c, and d, all containing real numbers;
the formulas for the mean and standard deviation
mean = (a+b+c+d)/4;
standard deviation = (((a-mean)**2 + (b-mean)**2 +
(c-mean)**2 + (d-mean)**2) / 3)**0.5

Compose: a let-in statement that returns an array of records, each containing the mean and standard deviation of one of the given sets of numbers.

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