Shortcut Trick
From p cos A = 2q sin A, we can write p = 2k sin A and q = k cos A.
Substitute these in 2p cosec A − q sec A = 3, using cosec A = 1 / sin A and sec A = 1 / cos A:
⇒ 2(2k sin A) / sin A − (k cos A) / cos A = 3
⇒ 4k − k = 3 ⇒ 3k = 3 ⇒ k = 1
⇒ p2 + 4q2 = (2(1) sin A)2 + 4(1 cos A)2 = 4 sin2 A + 4 cos2 A = 4(1) = 4
∴ The correct answer is 4.
Alternate Method
Given:
p cos A = 2q sin A --- (1)
2p cosec A − q sec A = 3 --- (2)
Formula Used:
cosec A = 1 / sin A
sec A = 1 / cos A
sin2 A + cos2 A = 1
Calculations:
From equation (1), we can express q in terms of p:
⇒ q = (p cos A) / (2 sin A)
Substitute the value of q in equation (2):
⇒ (2p / sin A) − [ (p cos A) / (2 sin A) ] × (1 / cos A) = 3
⇒ (2p / sin A) − (p / 2 sin A) = 3
⇒ (4p − p) / (2 sin A) = 3
⇒ 3p / 2 sin A = 3
⇒ p / 2 sin A = 1
⇒ p = 2 sin A
Now, substitute p = 2 sin A back into equation (1):
⇒ (2 sin A) cos A = 2q sin A
⇒ q = cos A
Now, calculate the value of p2 + 4q2:
⇒ p2 + 4q2 = (2 sin A)2 + 4(cos A)2
⇒ p2 + 4q2 = 4 sin2 A + 4 cos2 A
⇒ p2 + 4q2 = 4(sin2 A + cos2 A)
Since sin2 A + cos2 A = 1:
⇒ p2 + 4q2 = 4 × 1 = 4
∴ The correct answer is 4.
Additional Information
Pythagorean Identities
The most commonly used identity linking sine and cosine is sin2 θ + cos2 θ = 1, which holds true for all valid angles θ.
Reciprocal Trigonometric Ratios
The reciprocal of sine is cosecant (cosec θ = 1 / sin θ) and the reciprocal of cosine is secant (sec θ = 1 / cos θ).
Quotient Trigonometric Ratios
Tangent is the ratio of sine to cosine (tan θ = sin θ / cos θ) while cotangent is its reciprocal (cot θ = cos θ / sin θ).