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Question:

Let Ξ”PQR be a triangle. Let 𝐚 = QR, 𝐛 = RP. If |𝐚| = 12, |𝐛| = 4√3 and 𝐛.𝐜 = 24, then which of the following is (are) true?

|𝐚 Γ— 𝐛 + 𝐜 Γ— 𝐚| = 48√3

|𝐜|² + |𝐚| = 30

𝐚.𝐛 = 0

|𝐜|² - |𝐚| = 12

Solution:

Using vectors summation method, βˆ’πš = 𝐛 + 𝐜
|𝐚|Β² = |𝐜|Β² + |𝐛|Β² Β± 2|𝐜||𝐛|cosP
|𝐚|Β² = |𝐜|Β² + |𝐛|Β² + 2|𝐜||𝐛|cos(Ο€ βˆ’ P)
|𝐚|Β² = |𝐜|Β² + |𝐛|Β² βˆ’ 2𝐜 β‹… 𝐛
|𝐚|² = 12² = 144
(4√3)² = 48
144 = |𝐜|Β² + 48 βˆ’ 2(24)
144 = |𝐜|Β² + 48 βˆ’ 48
|𝐜|² = 144
|𝐜| = 12
Therefore, |𝐜|Β² βˆ’ |𝐚| = 144 βˆ’ 12 = 132 β‰  12
|𝐜|Β² + |𝐚| = 144 + 12 = 156 β‰  30
Since βˆ’πš = 𝐛 + 𝐜, then 𝐚 = βˆ’π› βˆ’ 𝐜
𝐚 β‹… 𝐛 = (βˆ’π› βˆ’ 𝐜) β‹… 𝐛 = βˆ’|𝐛|Β² βˆ’ 𝐜 β‹… 𝐛 = βˆ’48 βˆ’ 24 = βˆ’72 β‰  0
|𝐚 Γ— 𝐛 + 𝐜 Γ— 𝐚| = |𝐚 Γ— 𝐛 βˆ’ 𝐚 Γ— 𝐜| = |𝐚 Γ— (𝐛 βˆ’ 𝐜)| = |𝐚||𝐛 βˆ’ 𝐜|sinΞΈ
Since βˆ’πš = 𝐛 + 𝐜, then 𝐜 = βˆ’πš βˆ’ 𝐛
|𝐜| = 12
|𝐜|Β² βˆ’ |𝐚| = 144 βˆ’ 12 = 132
Option D is incorrect.
Let's check option A:
|𝐚 Γ— 𝐛 + 𝐜 Γ— 𝐚| = |𝐚 Γ— 𝐛 βˆ’ 𝐚 Γ— 𝐜| = |𝐚 Γ— (𝐛 βˆ’ 𝐜)| = |𝐚||𝐛 βˆ’ 𝐜|sinΞΈ
|𝐚 Γ— 𝐛| = |𝐚||𝐛|sin(Ο€ βˆ’ P) = |𝐚||𝐛|sinP
|𝐚 Γ— 𝐛 + 𝐜 Γ— 𝐚| = |𝐚 Γ— (𝐛 βˆ’ 𝐜)| = |𝐚||𝐛 βˆ’ 𝐜|sinΞΈ
|𝐚| = 12, |𝐛| = 4√3, 𝐜 β‹… 𝐛 = 24
|𝐜|² = 144
|𝐜| = 12
|𝐜|Β² βˆ’ |𝐚| = 144 βˆ’ 12 = 132
Only option D is true. However, the calculations show that none of the options are true.