Given seca1seca2+seca2seca3+....+secan−1secan.=k(tanan−tana1) ....(1)
Now, seca1seca2+seca2seca3+....+secan−1secan
1cosa1cosa2+1cosa2cosa3+.....1cosan−1cosan
=1sind(sin(a2−a1)cosa1cosa2+sin(a3−a2)cosa2cosa3+....sin(an−an−1)cosan−1cosan)
=1sind((tana2−tana1)+(tana3−tana2)+....+(tanan−tanan−1))
=1sind(tanan−tana1)
Comparing with (1), we get
k=1sind=2 where d=π6