Alan

Well, I'm not sure I understand it myself!

The first part says x is a member of D(f), though we're not told what D(f) means. I guessed it means the Domain of a function f. 

The backward, upper case E, means "there exists", so I simply guessed the whole thing means: if x is the set of variables in the domain of f there exists a specific value of x such that f(x) equals 6.

Still doesn't make a lot of sense though!!

I suspect the sum should be 255, rather than 256. If so everything works out nicely!

n = 1

\(\sum_{r=2}^{2014}(\frac{1}{\sum_{k=1}^rk})=\frac{2013}{2015}\\ \text{ add } \frac{2}{2015} \text{ to get } 1\)

Might be:

or:

if D stands for domain, this could mean something like:  "the set of x's belonging to the domain of the function f implies that there is an f(x) equal to 6".

0.25 is 1/4, so  81^0.25 is 81^(1/4)  This means the fourth root of 81.  That is, it is a number which, when raised to the power of 4 equals 81.

3^4 = 3*3*3*3 → 9*9 → 81

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v = u + a*t                  v = final velocity, u = intial velocity (1060 m/s here),

                                  a = acceleration (-50 m/s² here), t = time

v² = u² + 2a*s           s = distance

Is this what you are looking for?

e^ix = cos(x) + i*sin(x)      (where i = square root of -1)

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Easiest way to see this is to look at some numerical examples.  In the matrices below you can see that the LHS = RHS for all values of m for x₁, but LHS = RHS only for odd values of m for x₂.

(The first column of a matrix below is m, the second is the LHS of your original equation, the third is the RHS)

As follows:

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If we suppose point C should be (0, 7) then the centre of the circle lies at infinity, because the three points lie on a straight line!! 

Otherwise we must wait wait for the questioner to post corrected coordinates.