Expert: Paul Klarreich - 2/10/2010

Question
1-ABCD is a parallelogram, AB=8cm , BC=11cm ,BD=9cm find AC
also find the s.area of the parallelogram?
2-proof that: sin2A= 2tanA/1+tan^2A

Answer
Questioner: Christine
Country: Egypt
Category: Calculus
Private: No
Subject: Trignometry
Question: 1-ABCD is a parallelogram, AB=8cm , BC=11cm ,BD=9cm find AC
also find the s.area of the parallelogram?
2-proof that: sin2A= 2tanA/1+tan^2A
............................

1. Use the law of cosines to find angle A.  Then the area is

Area = AB * AD * sin A

2.

2tanA/(1+tan^2A)  << parenthesize carefully, next time.

=

2 s/c
-------------   << s means sin A, c means cos A
1 + s^2/c^2    


2 s c
--------------   << after clearing fractions.
c^2 + s^2

You can finish up.