1.

In the given diagram, AB is the diameter of the given circle with centre O. C and D are points on the circumference of the circle. If ∠ ABD =35° and ∠CDB =15° , then ∠CBD equals.(a) 55° (b) 75° (c) 40° (d) 25°

Answer»

(c) 40°

∠ADB =90° (Angle in a semi-circle) 

In ΔADB,∠DAB= 180°– (∠ADB+ ∠DBA) 

= 180° – (90° + 35°) = 180° – 125° = 55° (Angle sum prop. of a Δ)

In cyclic quad ABCD, 

∠A + ∠C = 180° ⇒∠C= 180° – 55° = 125° (Opp. ∠s of a cyclic quad. are supp.) 

∴ In ΔDCB,∠CBD = 180° – (15° + 125°) = 40°



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