cos<span> </span>5x<span> = </span>cos<span>(</span>4x<span> + </span>x<span>) = </span>cos<span> </span>x<span>*</span>cos<span> </span>4x<span> - sin x*sin </span>4x<span> </span>cos<span> </span>x<span>*</span>cos<span> </span>4x<span> - </span>cos<span> </span><em />5x<span> = </span>0<span> </span>cos<span> </span>x<span>*</span>cos<span> </span>4x<span> - </span>cosx<span>*</span>cos<span> </span>4x<span> + sin x*sin </span>4x<span> = </span>0<span> sin x*sin </span>4x<em /><span> = </span>0<span> sin </span>x<span> = </span>0<span>, x1 = Pi*k sin </span>4x<span> = </span>0<span>, </span>4x<span> = Pi*k</span>