The simplest formula isUn = (-8611*n^2 + 34477*n - 25082)/2 for n = 1, 2, 3.
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4, -1236, -108 is not a geometric system.
There are infinitely many polynomials of order 4 that will give these as the first four numbers and any one of these could be "the" explicit formula. There are also non-polynomial solutions. Short of reading the mind of the person who posed the question, there is no way of determining which of the infinitely many solutions is the "correct" one. For example, t(n) = (-17*n^4 + 170*n^3 - 575*n^2 + 830*n - 400)/4 for n = 1, 2, 3, ... The Simplest, though is t(n) = 5*n^2 - 5*n + 2 for n = 1, 2, 3, ...
xn=x1+(n-1)v<t xn=10+6(n-1) xn=4+6n
In linear algebra, Cramer's rule is an explicit formula for the solution of a system of linear equations with as many equations as unknowns, valid whenever the system has a unique solution.
The formula used to find the 99th term in a sequence is a^n = a^1 + (n-1)d. a^1 is the first term, n is the term number we wish to find, and d is the common difference. In order to find d, the pattern in the sequence must be determined. If the sequence begins 1,4,7,10..., then d=3 because there is a difference of 3 between each number. d can be quite simple or more complicated as it can be a function or formula in of itself. However, in the example, a^1=1, n=99, and d=3. The formula then reads a^99 = 1 + (99-1)3. Therefore, a^99 = 295.