Câu trả lời:
a) ta có : cos215+cos225+cos235+cos245+cos255+cos265+cos275cos215+cos225+cos235+cos245+cos255+cos265+cos275
=cos215+cos275+cos225+cos265+cos235+cos255+cos245=cos215+cos275+cos225+cos265+cos235+cos255+cos245 =cos215+cos2(90−15)+cos225+cos2(90−25)+cos235+cos2(90−35)+cos245=cos215+cos2(90−15)+cos225+cos2(90−25)+cos235+cos2(90−35)+cos245 =cos215+sin215+cos225+sin225+cos235+sin235+cos245=cos215+sin215+cos225+sin225+cos235+sin235+cos245
=1+1+1+12=72=1+1+1+12=72
b) ta có : sin210−sin220+sin230−sin240−sin250−sin270+sin280sin210−sin220+sin230−sin240−sin250−sin270+sin280
=sin210+sin280−sin220−sin270−sin240−sin250+sin230=sin210+sin280−sin220−sin270−sin240−sin250+sin230 =sin210+sin2(90−10)−sin220−sin2(90−20)−sin240−sin2(90−40)+sin230