Question

in an equilateral triangle ABC ,D is a point on side BC such that BD is equal to 1 by 3 BC prove that 9ADsquare equal to 7ABsquare​

Answer 1

Answer:

Step-by-step explanation:

Given :-

A ∆ABC in which AB = BC = CA and D is a point on BC such that BD = ⅓BC.

To prove :-

9AD² = 7AB² .

Construction :-

Draw AL ⊥ BC .

Proof :-

In right triangles ALB and ALC, we have

AB = AC ( given ) and AL = AL ( common )

∴ ∆ALB ≅ ∆ ALC [ By RHS axiom ] .

So, BL = CL .

Thus, BD = ⅓BC and BL = ½BC .

In ∆ALB, ∠ALB = 90° .

∴ AB² = AL² + BL² .......(1) [ by Pythagoras' theorem ] .

In ∆ALD , ∠ALD = 90° .

∴ AD² = AL² + DL² . [ by Pythagoras' theorem ] .

⇒ AD² = AL² + ( BL - BD )² .

⇒ AD² = AL² + BL² + BD² - 2BL.BD .

⇒ AD² = ( AL² + BL² ) + BD² - 2BL.BD .

⇒ AD² = AB² + BD² - 2BL.BD. [ using (1) ]

⇒ AD² = BC² + ( ⅓BC )² - 2( ½BC ). ⅓BC .

[ ∵ AB = BC, BD = ⅓BC and BL = ½BC ] .

⇒ AD² = BC² + 1/9BC² - ⅓BC² .

⇒ AD² = 7/9BC² .

⇒ AD² = 7/9AB² [ ∵ BC = AB ] .

9 AD square = 7 AB square..

Hence proved

Hope it helps you

Answer 2

may be useful mark as brainliest