Question

(2cos^x-5cosx+2)*sqrt(-7sinx)=0
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Answer 1

$(2cos^{2}x-5cosx+2)cdot sqrt{-7sinx}=0\left { {{2cos^{2}x-5cosx+2=0} atop {sinx=0}} right.\cosx=t\left { {{2t^{2}-5t+2=0} atop {sinx=0}} right.\D=25-16=9\t_{1}=2, t_{2}=frac{1}{2}\ left { {{cosx=frac{1}{2} } atop {sin x=0}} right. \ x=pm frac{pi}{3} +2pi n, n in Z ,\ x= pi k, k in Z$