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Mathematics: Post your doubts here!

qwertypoiu

qwertypoiu

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farhan141 said:
Oh alright.

So we have to ignore the 4 that is being multiplied with cos^2x and the other function.
Click to expand...
Yes. Even if you don't ignore the 4 you will get rid of it by dividing 4 on both sides and 0/4 is still 0. The other function was in terms of y so I decided to ignore it.
 
Shah34

Shah34

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Aoa can anyone solve the last part , plus also explain the concept thanks.
 

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unhellty

unhellty

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Referred to the origin O, the points A, B and C have position vectors given by
−−→ OA = i + 2j + 3k, −−→ OB = 2i + 4j + k and −−→ OC = 3i + 5j − 3k.
(i) Find the exact value of the cosine of angle BAC. [4]
(ii) Hence find the exact value of the area of triangle ABC. [3]
(iii) Find the equation of the plane which is parallel to the y-axis and contains the line through B and C. Give your answer in the form ax + by + cÏ = d. [5]

Please help me with (ii), how do I get the sin of the angle sqrt41/21 as referred to mark scheme? My friend told me to use A^2=B^2+C^2, but how can we know if triangle ABC is a right-angled triangle? Is there any other way to solve this besides my friends method and sine rule?

ASAP. Thanks!!
 
qwertypoiu

qwertypoiu

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unhellty said:
Referred to the origin O, the points A, B and C have position vectors given by
−−→ OA = i + 2j + 3k, −−→ OB = 2i + 4j + k and −−→ OC = 3i + 5j − 3k.
(i) Find the exact value of the cosine of angle BAC. [4]
(ii) Hence find the exact value of the area of triangle ABC. [3]
(iii) Find the equation of the plane which is parallel to the y-axis and contains the line through B and C. Give your answer in the form ax + by + cÏ = d. [5]

Please help me with (ii), how do I get the sin of the angle sqrt41/21 as referred to mark scheme? My friend told me to use A^2=B^2+C^2, but how can we know if triangle ABC is a right-angled triangle? Is there any other way to solve this besides my friends method and sine rule?

ASAP. Thanks!!
Click to expand...
If you know cosine of an angle you can easily find sine of it as well as follows:
sin^2(X) + cos^2(X) = 1
You probably know this identity already. Substitute what you know to find the other.
 
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