Cosine of 72 degrees (and constructing a regular pentagon)

By using (say) DeMoivre's theorem, we have that $\left( \cos\frac{2 \pi}{5}+ i \sin\frac{2\pi}{5} \right)^5=1$
Expanding the left side as the fifth power of a binomial, equating the imaginary parts on both sides of the equation, and then replacing  $\sin^2 72^\circ$ with  $1- \cos^2 72^\circ$

 $i\sin 72^\circ \left(5 \cos^4 72^\circ+10 \cos^2 72^\circ i^2 \sin ^2 72 ^\circ+ i^4 \sin^4 72^\circ \right)=0i$

 $16 \cos^4 72^\circ - 12 \cos^2 72^\circ  + 1=0$

Solving this quadratic in $\cos^2 72^\circ$, we get

  $$\cos^2 72^\circ  = \frac{12 \pm  \sqrt{80} } {32}$$

so

  $$\cos^2 72^\circ  = \frac{6 \pm 2 \sqrt{5} } {16}=\frac{\left( \sqrt{5}\pm 1 \right)^2}{4^2}$$

  $$\cos 72^\circ  = \pm \frac{\sqrt{5} \pm 1}{4}$$

where we can choose the correct value of the four possible values by noting that, because 72° is between 45° and 90°,  $\cos 72^\circ$ must lie between  $1 / \sqrt{2}$ and 0. Because  $\cos 72^\circ$ is positive, we choose the "+" before the fraction, and because  $\cos 72^\circ$ is less than $1 / \sqrt{2}$, which in turn is less than $\frac{\sqrt{5}+1}{4}$, we choose the "-" in the numerator:

$$\cos 72^\circ = \frac{\sqrt{5}-1}{4}$$

Constructing a regular pentagon

So we can construct $\cos 72^\circ$. For example, the diagonal of a 1-by-2 rectangle is $\sqrt{5}$. We could cut off one unit from a segment of length $\sqrt{5}$, then divide the segment of length $\sqrt{5}-1$ into four pieces of length $\frac{\sqrt{5} -1} {4}$. (Or we could construct the appropriate solution to the equation $4x^2 +2x -1 = 0$. See my post on Solving quadratic equations via geometric construction.)

Construct a unit circle centered at O, and construct a radius $\overline{OA}$.  Construct the point B on $\overline{OA}$ so that $\overline{OB}$ has length  $\cos 72^\circ$. If C is a point on the circle so that $\overline{BC}$ is perpendicular to $\overline{OA}$, then $\angle COA$ is a 72° angle, and both A and C are vertices of a regular pentagon inscribed in the circle.

What Math is Needed by All?

The (California version of the) Common Core State Standards in mathematics purport to be what all students need to be college and career ready.

The quantifier "all" in this context indicates that the math content should be the intersection (over all students) of math a student needs to be ready to begin college (or begin a career). Critics of the CCSSM who decry that the standards are not enough to prepare a student for an elite university such as Stanford are missing the point. The intent of the CCSS was never to include the union (over all students) of the math that a student needs to succeed in college. (And if the CCSS could provide all the math and English Language Arts that Stanford students need, then Stanford would not deserve its status as an elite school.)

And what do all students need? In 2013, the National Center on Education and the Economy released a study What Does It Really Mean to Be College and Work Ready?, reporting on both mathematics and English literacy. The report says, "Mastery of Algebra II is widely thought to be a prerequisite for success in college and careers. Our research shows that that is not so... Based on our data, one cannot make the case that high school graduates must be proficient in Algebra II to be ready for college and careers."

California's Intersegmental Committee of the Academic Senates (ICAS) represents the faculty academic senates of the three CA systems of higher education: the University of California (UC), the California State University (CSU), and the California Community College (CCC) system. The ICAS Statement on Competencies in Mathematics Expected of Entering College Students, revised in 2013, describes a number of mathematical topics that are or could be taught in high schools.

The ICAS competency statement describes mathematical subject matter in four categories: Part 1: Essential areas of focus for all entering college students, Part 2: Desirable areas of focus for all entering college students, Part 3: Essential areas of focus for students in quantitative majors, and Part 4: Desirable areas of focus for students in quantitative majors.

The mathematics that the CCSSM describe as what all students need should presumably match with what the ICAS statement describes as "essential" and lists in Part 1. But although the UC Board of Admissions and Relations with Schools (BOARS) states there is "close alignment" between the CCSS and the ICAS statement, the ICAS statement makes clear that there are many CCSS that are not "essential" but rather merely desirable or for only some students. Appendix B of the ICAS statement explicitly shows where Part 2, 3, and 4 areas of math are found in the CCSS (and NCTM standards).

And the Interim Environmental Scan Report to The Common Assessment Initiative Steering Committee has in  Appendix B a Table that shows a number of CCSS that do not occur at all in the ICAS statement.

Here are examples of CCSSM topics that might surprise some community college math faculty, especially those who believe that intermediate algebra as currently taught will be sufficient to cover all the CCSSM.

• Probability:  sample spaces, independent events, conditional probability, permutations and combinations; analyzing decisions and strategies using probability
• Statistics: assessing the fit of a function by plotting and analyzing residuals; interpreting the correlation coefficient of a linear model in context; normal distributions, random samples, estimating population parameters, simulations, using probability to make decisions
• Transformational geometry: congruence defined in terms of rigid motion; similarity defined in terms of dilations and rigid motions
• Trigonometry: trig ratios, special angles, 6 trig functions of real numbers; modeling periodic phenomena, proof and use of the Pythagorean trig identity $\cos^2 \theta + \sin^2 \theta = 1$