I need help? With proof for this one.

See below.

Step-by-step explanation:

ABC is an isosceles triangle with BA = BC.

That makes angles A and C congruent.

ABD is an isosceles triangle with AB = AD.

That makes angles ABD and ADB congruent.

Since m<ABD = 72 deg, then m<ADB = 72 deg.

Angles ADB and CDB are a linear pair which makes them supplementary.

m<ADB + m<BDC = 180 deg

72 deg + m<BDC = 180 deg

m<CDB = 108 deg

In triangle ABD, the sum of the measures of the angles is 180 deg.

m<A + m<ADB + m<ABD = 180 deg

m<A + 72 deg + 72 deg = 180 deg

m<A = 36 deg

m<C = 36 deg

In triangle BCD, the sum of the measures of the angles is 180 deg.

m<CBD + m<C + m<BDC = 180 deg

m<CBD + 36 deg + 108 deg = 180 deg

m<CBD = 36 deg

In triangle CBD, angles C and CBD measure 36 deg making them congruent.

Opposite sides DB and DC are congruent making triangle BCD isosceles.

cosine

step-by-step explanation:

step-by-step explanation:

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