If tan θ1. tan θ2=−a2b2 then the chord joining two points θ1 and θ2 on the ellipse x2a2+y2b2=1 will subtend a right angle at:
Centre
Let P be (θ1) and Q =(θ2)
Slope OP =ba tan θ1
and slope OQ =ba tan θ2
∴MOP×MOQ=b2a2×(−a2b2)=−1
∴ It subtends 90∘ at centre.