[Table
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio & Television News articles
I have already posted. As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
acknowledged. 
I
challenge you to find a calculus lesson in a modernday electronics magazine. In 1932, not all that long after Isaac
Newton developed differential calculus (that's a joke),
Radio News magazine ran a series of "Mathematics
in Radio" articles that included, among other topics, a few lessons in calculus. Anyone who has taken collegelevel
science or engineering courses knows how indispensible calculus is in working out many circuit, physics, and chemistry
problems. My appreciation for calculus came when I realized that it actually allowed me to derive the kinds of standard
equations that are commonly seen in lower level applications. For instance, if you needed to know the
volume of a sphere, you could look up the familiar
Volume = 4/3
π r
^{3}
formula, or you could write the equation
Volume = 
∫ 
2π 
∫ 
π 
∫ 
r 
ρ^{2} sin
Φ dρ
dΦ dθ
= 4/3 π r^{3}. 



0 
0 
0 
Area, mass, center of mass, and length of a spiral line, are other examples of problems that can be solved by knowing
the fundamental mathematics behind the equations.
See all available
vintage Radio News articles.
Mathematics in RadioCalculus
and its Application in Radio  Part Seventeen
By J. E. Smith*
In order to apply the formulas for
differentiating, the student should become familiar with the following examples with reference to the standard forms
(1)(5) inclusive (which were shown in the last lesson
^{+}).
Differentiate the following:
1) y = x
^{4}Here, formula (5) applies where x = v, and the exponent 4 = u.
Solution:
(A)
, but from the formula (1), dx/dx = 1,
therefore A becomes: dy/dx = 4x
^{3}
(answer).
2) y = x
^{6} 3) y = x
^{2}
4) y = x
^{9} 5) y  x
^{5}
6) y = ax
^{3} + bx
^{5}Here, formulas (3) and (5) apply, where a and b = c of formula
(3), and the exponents 3 and 5 respectively = n.
Solution:
= 3ax
^{2} + 5bx
^{4} (answer).
7) y = ax
^{4}
8) y = ax
^{6} + bx
^{2} 9) y = ax
^{7} 
x
^{6} 10) y  ax
^{3}  bx
^{3}
11) y = 5x
^{4} + 3x
^{2}  6
The derivative of a constant is zero; i.e., dc/dx = 0.
Solution:
= 20x
^{3} + 6x (answer).
12) y = 3cx
^{2}  8dx + 5e
13) y = 2ax
^{2} + 4dx
^{2}  5ax
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6 
The sinusoidal functions are those expressed by the sine and cosine curves as well as those functions which are
closely allied to them, which are the tangent and cotangent curves so important in geometry.
Electrical
and radio theory are very often advanced by the use of calculus, which takes the liberty of performing the necessary
operations on the sine and cosine functions. Let us apply the theory which was outlined above in finding the derivative
of a sine function. That is, if a sine function is expressed by y = sin x, it is the purpose of this analysis to
determine its proper derivative.
In the previous texts it has been shown how to plot the function y = sin
x, and this is shown in Figure 1 for the values of x from 0180 degrees. In order to study the derivative of this
function, reference is made to Figures 2 and 3, where the values of x have been plotted from 090 degrees. Applying
the theory which has been outlined above, we see that when Δx approaches zero, Δy/Δx finally approaches
a limiting value, and it is remembered that this limiting value is called the derivative of y with respect to x,
This limit, represented by dy/dx, has been shown above to be equal to the tangent θ.
With reference
to Figure 4, it is noticed that the angle θ at the point A is somewhat larger than the values of θ for the points
Band C. Finally, at the point D, the tangent line TD becomes parallel to the abscissa, and it is noticed that the
angle θ has become increasingly smaller until it has approached the value of zero.
Now, the derivative of
the function y = sin x at the point A is by definition equal to the tangent of θ. In like manner, the derivatives
of the function at points B, C and D are again equal respectively to the tangents of the angles θ.
By close
analysis of the relative sizes of these angles, we arrive at the following table:
For point A, θ equals
about 45°
For point B, θ equals about 35°
For point C, θ equals about 17°
For point D, θ
equals about 0°
But the tangent of these angles is by definition equal to the derivative of the function
at its respective points; thus:
dy/dx at point A is about 1
dy/dx at point B is about 0.70
dy/dx at point C is about 0.31
dy/dx at point D is about 0
Plotting these values of dy/dx at
point A, B and C, respectively, we have the graph of Figure 5. But this dy/dx curve is further noticed to be the
same as the function y = cos x, so it is apparent that:
d (sin x)/ax = cos x
or the derivative of
the sin x is equal to the cos x.
Figure 6 shows the functions y = sin x and y = cos x plotted together.
Formulas for Differentiating Standard Forms The following formulas are a continuation
of the ones listed above from (15) inclusive.
6. d/dx (sin v) = cos v (dv/dx)
The derivative
of a sine function is equal to its cosine function, times the derivative of the function.
7. d (cos
v) / dx = sin v (dv/dx)
8. d (tan v) / dx = sec
^{2} v (dv/dx)
9. d (cot v) /
dx = csc
^{2} v (dv/dx)
See v and csc v above are the abbreviations for secant and cosecant, respectively.
The application of the above differential forms to radio theory is now quite readily presented.
Answers to Problems: 1) 4x
^{3} 2) 6x
^{5}
3) 2x
4) 9x
^{8} 5) 5x
^{4} 6) 3ax
^{2}
+ 5bx
^{4} 7) 4ax
^{3} 8) 6ax
^{5} + 2bx
9)
7ax
^{6}  6x
^{5}10) 1  3ax
^{2}  3bx
^{2}11) 20x
^{3}
+ 6x
12) 6cx  8d
13) 4ax + 8x  5a
*President National Radio
Institute.
+Note  Many of the examples have been taken from the book, "Elements of the Differential and
Integral Calculus,"
by W. A.
Granville, published by Ginn and Company, N. Y. Posted August
22, 2013