Mathematics
In triangle ABC, AD is perpendicular to side BC and AD2 = BD × CD.
prove that angle BAC = 90°.
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Answer
Triangle ABC is shown in the figure below:

Given :
AD2 = BD × CD
⇒
∠ADB = ∠ADC [Both = 90°]
∴ △DBA ~ △DAC (By SAS).
Since, triangles are similar they will be equiangular.
∴ ∠1 = ∠C and ∠2 = ∠B
⇒ ∠1 + ∠2 = ∠B + ∠C
⇒ ∠A = ∠B + ∠C
By angle sum property :
⇒ ∠A + ∠B + ∠C = 180°
⇒ ∠A + ∠A = 180°
⇒ 2∠A = 180°
⇒ ∠A = 90°.
From figure,
⇒ ∠BAC = ∠A = 90°.
Hence, proved that ∠BAC = 90°.
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