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Question

The value of sec1(1410k=0sec(7π12+kπ2)sec(7π12+(k+1)π2)) in the interval [π4,3π4] equals

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Solution

sec11410k=01cos(7π12+kπ2)cos(7π12+(k+1)π2)

sec11410k=0sin[(7π12+(k+1)π2) (7π12+kπ2)]cos(7π12+kπ2)cos(7π12+(k+1)π2)
sin(AB)=sinAcosBcosAsinB

sec11410k=0(sin(7π12+(k+1)π2)cos(7π12+kπ2))(cos(7π12+(k+1)π2)sin(7π12+kπ2))cos(7π12+kπ2)cos(7π12+(k+1)π2)

sec1(1410k=0(tan(7π12+(k+1)π2))(tan(7π12+kπ2)))

sec114(tan((7π12+π2)tan(7π12)+(tan(7π12+2π2)tan(7π12+π2)+ ....+(tan(7π12+11π2)tan(7π12+10π2))

sec1(14(tan(7π12+11π2)(tan7π12)))

sec1(14tan(6π+π12)(tan7π12))

sec1(14(tanπ12)(tan7π12))

sec1(14((23)+(2+3)))

sec1(1)=0

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