[H] sin²(k°)+sin²(90°−k°)=1. Pairs: (10,80),(20,70),(30,60),(40,50)=4 pairs each=1. Plus sin²(90°)... wait: 10,20,30,40,50,60,70,80,90. Pairs: (10,80),(20,70),(30,60),(40,50)=4 pairs giving 4×1=4. Plus sin²(90°)... wait 9 terms total: sin²45° is alone? No: 9 terms: 10°,20°,...,90°. Pairs: (10,80),(20,70),(30,60),(40,50)=8 terms. Plus 90°: sin²90°=1. Plus 50° already counted. Actually: 9 terms pairing: 10+80=90✓, 20+70=90✓, 30+60=90✓, 40+50=90✓ → 4 pairs. Plus 90° alone. 4(1)+1=5. Index 1.