A=(sin210+sin280)+(sin220+sin70)+(sin230+sin260)+(sin240+sin250)
Lại có: sin80=cos10; sin70=cos20; sin60=cos30; sin50=cos40
=> sin280=cos210; sin270=cos220; sin260=cos230; sin250=cos240
=>A=(sin210+cos210)+(sin220+cos220)+(sin230+cos230)+(sin240+cos240)
=>A=1+1+1+1=4
A = 4
học tốt nha