If cos-1x2+cos-1y3=θ, then 9x2-12xycosθ+4y2=

AP EAMCET · Maths · Inverse Trigonometric Functions

If $\cos^{- 1} (\frac{x}{2}) + \cos^{- 1} (\frac{y}{3}) = θ,$ then $9 x^{2} - 12 x y \cos θ + 4 y^{2} =$

A $36 \sin^{2} θ$
B $37 \sin^{2} θ$
C $39 \sin^{2} θ$
D $36 \cos^{2} θ$

Verified Solution

Answer & Solution

Correct Answer

(A) $36 \sin^{2} θ$

Step-by-step Solution

Detailed explanation

Given: cos-1x2+cos-1y3=θ ⇒cos-1x2y3-1-x241-y29=θ ⇒x2y3-4-x249-y29=cosθ ⇒xy6-4-x229-y23=cosθ ⇒xy-4-x2·9-y2=6cosθ ⇒4-x2·9-y2=xy-6cosθ Squaring both sides, we get 4-x2·9-y2=36cos2θ+x2y2-12xycosθ…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

Same subject

Explore more questions on app

From AP EAMCET

Match the following?

	Column $- I$		Column $- II$
(a)	Ratio of change in time period of a simple pendulum with temperature to its original time period	(i)	$α Δ T$
(b)	Ratio of the value of a length to its scale reading	(ii)	$T$
(c)	Reciprocal of coefficient of volume expansion for an ideal gas of constant pressure	(iii)	$(1 + α Δ T)$
(d)	$\frac{F}{Y A} =$	(iv)	$\frac{1}{2} α Δ T$

AP EAMCET 2021 Medium

Explore more questions on app