Solve the equation: \sin^{2000}x+\cos^{2000}x=1

Gary Johnson

Gary Johnson

Answered question

2022-02-26

Solve the equation:
sin2000x+cos2000x=1

Answer & Explanation

blokova5u8

blokova5u8

Beginner2022-02-27Added 7 answers

Since we are concerned with the case when θkπ2, we get
0<sin2θ<1
and
0<cos2θ<1
Now observe that for 0<θ<π2
1=(sin2θ+cos2θ)2=sin4+cos4θ+2sin2θcos2θ>sin4θ+cos4θ
Now observe that for 0<x<1,
xn+(1x)n<1
for n2, and it is monotonically strictly decreasing in n.
To show this, verify it for n=2, then use induction. The inductive step being:
1>xn+(1x)n=(xn+(1x)n)(x+1x)=xn+1+(1x)n+1+xn(1x)+(1x)nx>xn+1+(1x)n+1
Now put x=sin2θ and you are done. θ=kπ2 give the only solutions.

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