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Len [333]
3 years ago
9

Find all solutions of the equation in the interval [0,2π) sec(theta) +2=0

Mathematics
1 answer:
Komok [63]3 years ago
6 0
I will first reveal the most obvious secret of trig: Most of the problems are 30/60/90 or 45/45/90 triangles of some sort.

\sec \theta + 2  = 0

\dfrac{1 }{ \cos \theta} = -2

\cos \theta = - \frac 1 2

The rule to remember is \cos x = \cos a has solutions

x = \pm a + 2 \pi k \quad integer k

Continuing where we left off, a cosine of -1/2 is a 30/60/90 triangle in the second or third quadrant; we pick second, and a little thought tells us the angle is  120^\circ.

\cos \theta = - \frac 1 2

\cos \theta = \cos \frac {2\pi} 3

\theta = \pm \frac {2\pi} 3 + 2 \pi k \quad integer k

In the range we want, that's k=0 with the plus sign and k=1 with the minus, so 

\theta = \frac {2\pi} 3 or -\frac {2\pi} 3 + 2 \pi = \frac{4 \pi} 3

\theta = \frac {2\pi} 3 or \frac{4 \pi} 3
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