Question

x2) Triangle ABC is an isosceles triangle with BA = BC 72° D lies on AC ABD is an isosceles triangle with AB = AD Angle ABD = 72° 36 A D Show that triangle BCD is isosceles. You must give a reason for each stage of your working. ​

Answer 1

An isosceles triangle is one with two equal-length sides. The ΔBCD is an isosceles triangle.

What is an isosceles triangle?

An isosceles triangle is one with two equal-length sides. It is sometimes stated as having exactly two equal-length sides, and sometimes as having at least two equal-length sides, with the latter form containing the equilateral triangle as a particular case.

Given ΔABC is an isosceles triangle with side AB = AC. Therefore, as per the base angle theorem,

∠A = ∠C = 36°

Also, in ΔABC, the sum of the angles can be written as,

∠ABC + ∠BAC + ∠BCA = 180°

∠ABC + 36° + 36° = 180°

∠ABC = 108°

For ∠ABC we can write,

∠ABC = ∠ABD + ∠CBD

108° = 72° + ∠CBD

∠CBD = 36°

Now, in ΔBCD, the measure of ∠CBD and ∠DCB both equals to 36°. Therefore, we can write, that the ΔBCD is an isosceles triangle.

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