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Differentiate w.r.t. n
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x ^ 2 -3x +70 = 0
Quadratic equations such as this one can be solved by a new direct factoring method that does not require guess work. To use the direct factoring method, the equation must be in the form x^2+Bx+C=0.
r + s = 3 rs = 70
Let r and s be the factors for the quadratic equation such that x^2+Bx+C=(x−r)(x−s) where sum of factors (r+s)=−B and the product of factors rs = C
r = \frac{3}{2} - u s = \frac{3}{2} + u
Two numbers r and s sum up to 3 exactly when the average of the two numbers is \frac{1}{2}*3 = \frac{3}{2}. You can also see that the midpoint of r and s corresponds to the axis of symmetry of the parabola represented by the quadratic equation y=x^2+Bx+C. The values of r and s are equidistant from the center by an unknown quantity u. Express r and s with respect to variable u. <div style='padding: 8px'><img src='https://opalmath.azureedge.net/customsolver/quadraticgraph.png' style='width: 100%;max-width: 700px' /></div>
(\frac{3}{2} - u) (\frac{3}{2} + u) = 70
To solve for unknown quantity u, substitute these in the product equation rs = 70
\frac{9}{4} - u^2 = 70
Simplify by expanding (a -b) (a + b) = a^2 – b^2
-u^2 = 70-\frac{9}{4} = \frac{271}{4}
Simplify the expression by subtracting \frac{9}{4} on both sides
u^2 = -\frac{271}{4} u = \pm\sqrt{-\frac{271}{4}} = \pm \frac{\sqrt{271}}{2}i
Simplify the expression by multiplying -1 on both sides and take the square root to obtain the value of unknown variable u
r =\frac{3}{2} - \frac{\sqrt{271}}{2}i = 1.500 - 8.231i s = \frac{3}{2} + \frac{\sqrt{271}}{2}i = 1.500 + 8.231i
The factors r and s are the solutions to the quadratic equation. Substitute the value of u to compute the r and s.
2n^{2-1}-3n^{1-1}
The derivative of a polynomial is the sum of the derivatives of its terms. The derivative of a constant term is 0. The derivative of ax^{n} is nax^{n-1}.
2n^{1}-3n^{1-1}
Subtract 1 from 2.
2n^{1}-3n^{0}
Subtract 1 from 1.
2n-3n^{0}
For any term t, t^{1}=t.
2n-3
For any term t except 0, t^{0}=1.