Question

If the angle of vertex of an isosceles triangle is 100∘, then the measure of a base angle equals .  40∘80∘20∘

Solution

The correct option is A 40∘ Given that ∠A=100∘ Since △ABC is isosceles, we have ∠ABC=∠ACB=x   (say) Now, ∠x+∠x+100∘=180∘   (Using angle sum property) ⟹2x+100∘=180∘ ⟹2x=80∘⟹x=40∘

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