[TEX] sin^8x + cos^8x[/TEX]
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VT[tex]=(sin^4x+cos^4x)^2-2sin^4xcos^4x[/tex]
[tex]=[(sin^2x+cos^2x)^2-2sin^2xcos^2x]^2- \frac{1}{8}.sin^42x[/tex]
[tex]=(1-\frac{1}{2}sin^22x)^2-\frac{1}{8}.sin^42x[/tex]
[tex]=1-sin^22x+\frac{1}{8}.sin^42x[/tex]
[tex]=1-\frac{1-cos4x}{2}+\frac{1}{8}.\frac{(1-cos4x)^2}{4}[/tex]
[tex]=1-\frac{1-cos4x}{2}+\frac{1}{32}.(1-cos4x+cos^24x)[/tex]
[tex]=\frac{32-16+16cos4x+1-2cos4x+cos^24x}{32}[/tex]
[tex]=\frac{32-16+16cos4x+1-2cos4x+\frac{1+cos8a}{2}}{32}[/tex]
[tex]=\frac{35}{64}+\frac{7}{16}cos4x+\frac{1}{64}cos8x[/tex]