2:
b: \(A=cos^228^0+cos^241^0+cos^262^0+cos^249^0\)
\(=\left(cos^228^0+cos^262^0\right)+\left(cos^241^0+cos^249^0\right)\)
\(=\left(\sin^262^0+cos^262^0\right)+\left(\sin^241^0+cos^241^0\right)\)
=1+1
=2
c: \(1-2\cdot\sin\alpha\cdot cos\alpha=0\)
=>\(1-\sin2\alpha=0\)
=>\(\sin2\alpha=1\)
=>\(2\alpha=\frac{\pi}{2}+k2\pi\)
=>\(\alpha=\frac{\pi}{4}+k\pi\)
mà \(0^0<\alpha<90^0\)
nên \(\alpha=45^0\)
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