Gọi $M= \cos^{2} 10^{\circ}+\cos^{2} 20^{\circ}+\cos^{2} 30^{\circ}+\cos^{2} 40^{\circ}+\cos^{2} 50^{\circ}+\cos^{2} 60^{\circ}+\cos^{2} 70^{\circ}+\cos^{2} 80^{\circ}$ thì $M$ bằng?
4
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