sec−1⎛⎜⎝1410∑k=01cos(7π12+kπ2)cos(7π12+(k+1)π2)⎞⎟⎠
sec−1⎛⎜⎝1410∑k=0sin[(7π12+(k+1)π2)− (7π12+kπ2)]cos(7π12+kπ2)cos(7π12+(k+1)π2)⎞⎟⎠
∵sin(A−B)=sinA⋅cosB−cosA⋅sinB
sec−1⎛⎜⎝1410∑k=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)⎞⎟⎠
sec−1(1410∑k=0(tan(7π12+(k+1)π2))−(tan(7π12+kπ2)))
sec−114(tan((7π12+π2)−tan(7π12)+(tan(7π12+2π2)−tan(7π12+π2)+ ....+(tan(7π12+11π2)−tan(7π12+10π2))
sec−1(14(tan(7π12+11π2)−(tan7π12)))
sec−1(14tan(6π+π12)−(tan7π12))
sec−1(14(tanπ12)−(tan7π12))
sec−1(14((2−√3)+(2+√3)))
sec−1(1)=0