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Question : In $\triangle $ABC, AD$\perp$ BC and AD2 = BD × DC. The measure of $\angle$ BAC is:

Option 1: 60°

Option 2: 75°

Option 3: 90°

Option 4: 45°


Team Careers360 19th Jan, 2024
Answer (1)
Team Careers360 21st Jan, 2024

Correct Answer: 90°


Solution :
Given: In $\triangle$ABC, AD$\perp$BC and AD2 = BD × DC.
Applying Pythagoras theorem in $\triangle$ADB and $\triangle$ADC,
AB2 = AD2 + BD2 ____(i)
AC2 = AD2 + DC2 ____(ii)
Adding equation (i) and (ii),
AB2 + AC2 = AD2 + BD2 + AD2 + DC2 
AB2 + AC2 = 2AD2 + BD2 + DC2 
AB2 + AC2 = 2 BD × DC + BD2 + DC2 
AB2 + AC2 = (BD + DC)2 
AB2 + AC2 = BC2 
$\triangle$ABC is a right-angled triangle.
$\angle$BAC = 90°
Hence, the correct answer is 90°.

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