Class 12 RD Sharma Solutions – Chapter 25 Vector or Cross Product – Exercise 25.1 | Set 3
Question 25. If, and , find
Solution:
We know that,
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As ,
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Thus,
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Question 26. Find the area of the triangle formed by O, A, B when,
Solution:
The area of a triangle whose adjacent sides are given by and is
=>
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=> Area =
=> Area =
=> Area =
=> Area =
=> Area = square units.
Question 27. Let , and. Find a vector which is perpendicular to bothand and
Solution:
Given that is perpendicular to both and .
=> ……….(1)
=> ……….(2)
Also,
=> …….(3)
Let
From eq(1),
=> d1 + 4d2 + 2d3 = 0
From eq(2),
=> 3d1 – 2d2 + 7d3 = 0
From eq(3),
=> 2d1 – d2 + 4d3 = 15
On solving the 3 equations we get,
d1 = 160/3, d2 = -5/3, and d3 = -70/3,
=>
Question 28. Find a unit vector perpendicular to each of the vectors and , where and .
Solution:
Given that, and
Let
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Let
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A vector perpendicular to both and is,
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To find the unit vector,
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Question 29. Using vectors, find the area of the triangle with the vertices A(2, 3, 5), B(3, 5, 8), and C(2, 7, 8).
Solution:
Given, A(2, 3, 5), B(3, 5, 8), and C(2, 7, 8)
Let,
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Then,
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The area of a triangle whose adjacent sides are given by and is
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=> Area =
=> Area =
=> Area = √61/2
Question 30. If , , are three vectors, find the area of the parallelogram having diagonals and .
Solution:
Given, , ,
Let,
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The area of the parallelogram having diagonals and is
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=> Area =
=> Area =
=> Area =
=> Area = √21/2
Question 31. The two adjacent sides of a parallelogram are and . Find the unit vector parallel to one of its diagonals. Also, find its area.
Solution:
Given a parallelogram ABCD and its 2 sides AB and BC.
By triangle law of addition,
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Unit vector is,
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Area of a parallelogram whose adjacent sides are given is
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Thus area is,
=> Area =
=> Area =
=> Area =
=> Area = 11 √5 square units
Question 32. If either or , then . Is the converse true? Justify with example.
Solution:
Let us take two parallel non-zero vectors and
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For example,
and
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But,
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Hence the converse may not be true.
Question 33. If , and, then verify that .
Solution:
Given, , and
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=> …..eq(1)
Now,
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And,
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Thus,
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=> …eq(2)
Thus eq(1) = eq(2)
Hence proved.
Question 34(i). Using vectors find the area of the triangle with the vertices A(1, 1, 2), B(2, 3, 5), and C(1, 5, 5).
Solution:
Given, A(1, 1, 2), B(2, 3, 5), and C(1, 5, 5)
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Now 2 sides of the triangle are given by,
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Area of the triangle whose adjacent sides are given is
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Thus area of the triangle is,
=> Area =
=> Area =
=> Area = √61/2
Question 34(ii). Using vectors find the area of the triangle with the vertices A(1, 2, 3), B(2, -1, 4), and C(4, 5, -1).
Solution:
Given, A(1, 2, 3), B(2, -1, 4), and C(4, 5, -1)
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Now 2 sides of the triangle are given by,
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Area of the triangle whose adjacent sides are given is
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Thus area of the triangle is,
=> Area =
=> Area =
=> Area = √274/2
Question 35. Find all the vectors of magnitude that are perpendicular to the plane of and .
Solution:
Given, and
A vector perpendicular to both and is,
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Unit vector is,
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Now vectors of magnitude are given by,
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=> Required vectors,
Question 36. The adjacent sides of a parallelogram are and . Find the 2 unit vectors parallel to its diagonals. Also, find its area of the parallelogram.
Solution:
Given, and
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Unit vector is,
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Area is given by ,
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