Ta có:
sin100=sin80
sin110=sin70
.................
sin170=sin10
sin180=sin0=0=>(sin 180)^2=0
=>S(tổng đã cho)=2.[(sin10)^2+(sin20)^2+.....+(sin80)^2]+(sin90)^2+(sin180)^2
=>S=2.[(sin10)^2+(sin20)^2+.....+(sin80)^2]+1 (do sin90=1)
Lại có:
(sin10)^2=(cos80)^2
(sin20)^2=(cos70)^2
(sin30)^2=(cos60)^2
(sin40)^2=(cos50)^2
=>(sin10)^2+(sin20)^2+.....+(sin80)^2 = [(cos80)^2 + (sin80)^2]+....+[(cos50)^2 + (sin50)^2]=4
Vậy S=2.4+1=9