Book: RD Sharma - Mathematics (Volume 1)
Chapter: 4. Inverse Trigonometric Functions
Subject: Maths - Class 12th
Q. No. 1 of Exercise 4.13
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1
If cos–1 x/2 + cos–1y/3 = a, then prove that 9x2–12xy cos a + 4y2 = 36 sin2 a.
Given:- 
Take
We know that, Formula
Thus,
⇒ 
⇒ 
⇒ 
⇒ 
Now lets take square of both side, we get
⇒ 
⇒ 
⇒ 
⇒ 
⇒ 
⇒ 
Hence Proved
