\(c,cos^220^o+cos^250^o+cos^270^o+cos^240^o.\\ =cos^2\left(90^o-70^o\right)+cos^250^o+cos^270^o+cos^2\left(90^o-50^o\right).\\ =sin^270^o+cos^250^o+cos^270^o+sin^250^o.\\ =\left(sin^270^o+cos^270^o\right)+\left(cos^250^o+sin^250^o\right).\\ =1+1=2.\)
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