13. cos 0 + cos(x+2)+cos (21+0)+ +c...
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13. cos 0 + cos(x+2)+cos (21+0)+ +cos(207+0) (a) O (b) cos 0 (c) sine (d) -cos o

JEE/Engineering Exams
Maths
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( begin{array}{l}text { Solve } qquad begin{aligned} & cos theta+cos (pi+theta)+cos (2 pi+theta)+cdots+operatorname{Cos}(2 pi+theta) =& cos theta-cos theta+cos theta-cos theta+cdots+cos (19 pi+theta) =& cos theta-cos theta+cos theta-cos theta+cdots+cos theta+cos theta =& cos theta end{aligned} qquad= & operatorname{or} operatorname{tin} n(1)end{array} )
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