(2)cos21°+cos25°+cos29°+…+cos285°+cos289° In the given expression we can seethat the angles are 1° , 5° , 9° , 13° ,…, 81° , 85° , 89° Let the number of terms in theexpression be n. ∴a+(n–1)d=tn ⇒ 1 + (n – 1)4 = 89 ⇒ n = 23 ∴cos21°+cos25°+cos29°+…+cos285°+cos289° =cos21°+cos289°+cos25°+cos285°+…+cos237°+cos253°+cos241°+cos249°+cos245° =cos21°+sin21°+cos25°+sin25°+…+cos241°+sin241°+cos245° =[1+1+1+…+1](11times)+(