The Trick To Converting Radians To Degrees

Geometry Level 2

There are a couple ways to convert from radians to degrees, but Jacob claims that he has found a new method to do this conversion that is perhaps faster than all the others. His method is simple: Given a radian measure in simplest form, substitute 180 180 in for π \pi and then simplify the expression.

For example, if Jacob wanted to convert 2 π 5 \frac{2\pi}{5} radians to degrees, he would put 180 180 in place of π \pi and then simplify the numerical expression (And then, of course, label his expression with a degree symbol). You can see him use this method on the example below:

2 π 5 2 ( 180 ) 5 = 360 5 = 72 7 2 \frac{2\pi}{5}\rightarrow \frac{2(180)}{5}=\frac{360}{5}=72\rightarrow 72^{\circ}

Of course, Jacob believes that his trick will work as long as π \pi is visible in the simplified expression. For example, Jacob knows he can use his trick to convert π 3 \frac{\pi}{3} radians but not 2 2 radians because no substitution can be done.

As long as π \pi is visible in the simplified expression, does Jacob's trick always work?

No Yes

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1 solution

PiMan Seys
Jul 16, 2018

Let's look at a different scenario when π \pi is visible in the simplified expression. We've seen it used in the numerator, but what happens when π \pi in the denominator? Does it still work?

Let's see by first converting 1 π \frac{1}{\pi} radians into degrees the normal way (Using dimensional analysis), and then convert using Jacob's trick. If the results are not equal, Jacob's trick would no longer hold.

Like I said, let's first convert using dimensional analysis.

1 r a d π × 18 0 π r a d \frac{1 \;rad}{\pi}\times \frac{180^{\circ}}{\pi \;rad}

Of course, the radian labels cancel out like common factors when multiplying fractions.

1 π × 18 0 π = 18 0 π 2 \frac{1}{\pi}\times \frac{180^{\circ}}{\pi}=\frac{180^{\circ}}{\pi^{2}}

So, we know that 1 π \frac{1}{\pi} radians is equal to 180 π 2 \frac{180}{\pi^{2}} degrees (Note that 18 0 π 2 \frac{180^{\circ}}{\pi^{2}} is equivalent to saying ( 180 π 2 ) (\frac{180}{\pi^{2}})^{\circ} ).

Let's now try using Jacob's trick.

1 π 1 ( 180 ) ( 1 180 ) \frac{1}{\pi}\rightarrow \frac{1}{(180)}\rightarrow (\frac{1}{180})^{\circ}

So, using Jacob's trick, our answer becomes 1 180 \frac{1}{180} degrees. Are the two answers equivalent?

Well, no. 180 π 2 18.238 \frac{180}{\pi^{2}}\approx 18.238 and 1 180 0.006 \frac{1}{180}\approx 0.006 . So, it is quite clear that 180 π 2 1 180 \frac{180}{\pi^{2}}^{\circ}\neq\frac{1}{180}^{\circ} and therefore Jacob's trick doesn't always work . In fact, this trick always works when π \pi is only on the numerator, but it doesn't usually work when π \pi is in the denominator.

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