Answer: 2x-1
combine like terms: 4x and -2x
-4 and 3
Not sure why such an old question is showing up on my feed...
Anyway, let
![x=\tan^{-1}\dfrac43](https://tex.z-dn.net/?f=x%3D%5Ctan%5E%7B-1%7D%5Cdfrac43)
and
![y=\sin^{-1}\dfrac35](https://tex.z-dn.net/?f=y%3D%5Csin%5E%7B-1%7D%5Cdfrac35)
. Then we want to find the exact value of
![\cos(x-y)](https://tex.z-dn.net/?f=%5Ccos%28x-y%29)
.
Use the angle difference identity:
![\cos(x-y)=\cos x\cos y+\sin x\sin y](https://tex.z-dn.net/?f=%5Ccos%28x-y%29%3D%5Ccos%20x%5Ccos%20y%2B%5Csin%20x%5Csin%20y)
and right away we find
![\sin y=\dfrac35](https://tex.z-dn.net/?f=%5Csin%20y%3D%5Cdfrac35)
. By the Pythagorean theorem, we also find
![\cos y=\dfrac45](https://tex.z-dn.net/?f=%5Ccos%20y%3D%5Cdfrac45)
. (Actually, this could potentially be negative, but let's assume all angles are in the first quadrant for convenience.)
Meanwhile, if
![\tan x=\dfrac43](https://tex.z-dn.net/?f=%5Ctan%20x%3D%5Cdfrac43)
, then (by Pythagorean theorem)
![\sec x=\dfrac53](https://tex.z-dn.net/?f=%5Csec%20x%3D%5Cdfrac53)
, so
![\cos x=\dfrac35](https://tex.z-dn.net/?f=%5Ccos%20x%3D%5Cdfrac35)
. And from this,
![\sin x=\dfrac45](https://tex.z-dn.net/?f=%5Csin%20x%3D%5Cdfrac45)
.
So,
If <em>z</em> ⁷ = 128<em>i</em>, then there are 7 complex numbers <em>z</em> that satisfy this equation.
![z^7 = 128i = 2^7i = 2^7e^{i\frac\pi2}](https://tex.z-dn.net/?f=z%5E7%20%3D%20128i%20%3D%202%5E7i%20%3D%202%5E7e%5E%7Bi%5Cfrac%5Cpi2%7D)
![\implies z=\sqrt[7]{2^7} e^{i\frac17\left(\frac\pi2+2n\pi\right)}](https://tex.z-dn.net/?f=%5Cimplies%20z%3D%5Csqrt%5B7%5D%7B2%5E7%7D%20e%5E%7Bi%5Cfrac17%5Cleft%28%5Cfrac%5Cpi2%2B2n%5Cpi%5Cright%29%7D)
(where <em>n</em> = 0, 1, 2, …, 6)
![\implies z = 2 e^{i\left(\frac\pi{14}+\frac{2n\pi}7\right)}](https://tex.z-dn.net/?f=%5Cimplies%20z%20%3D%202%20e%5E%7Bi%5Cleft%28%5Cfrac%5Cpi%7B14%7D%2B%5Cfrac%7B2n%5Cpi%7D7%5Cright%29%7D)
![\displaystyle\implies z = 2 \left(\cos\left(\frac\pi{14}+\frac{2n\pi}7\right)+i\sin\left(\frac\pi{14}+\frac{2n\pi}7\right)\right)](https://tex.z-dn.net/?f=%5Cdisplaystyle%5Cimplies%20z%20%3D%202%20%5Cleft%28%5Ccos%5Cleft%28%5Cfrac%5Cpi%7B14%7D%2B%5Cfrac%7B2n%5Cpi%7D7%5Cright%29%2Bi%5Csin%5Cleft%28%5Cfrac%5Cpi%7B14%7D%2B%5Cfrac%7B2n%5Cpi%7D7%5Cright%29%5Cright%29)
Answer:
S
Step-by-step explanation: