1. In this question, you will be using the following trigonometric identities: cos? cos(a + B) sin(a + B) a + sin? a = 1 (1) (2) cos a cos B – sin a sin ß sin a cos B + cos a sin B where a, B E R. You do not need to prove these identities. You may also use without proof the fact that the set cos a : α R sin a is exactly the set of unit vectors in R². Now for any real number a, define cos a – sin a sin a R. = COS Q (a) Prove that for all a, ß E R, RaR3 = Ra+8 (b) Using part (a), or otherwise, prove that Ra is invertible and that R1 all a E R. = R_a, for (c) Prove that for all a E R and all x, y e R², (Rax) · (Ray) =x • y

Advanced Engineering Mathematics
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Chapter2: Second-order Linear Odes
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1. In this question, you will be using the following trigonometric identities:
cos? a + sin? a
(1)
(2)
(3)
1
cos(a + B)
sin(a + B)
cos a cos B – sin a sin B
sin a cos B + cos a sin 3
where a, B E R. You do not need to prove these identities. You may also use without
proof the fact that the set
cos a
: αER
sin a
is exactly the set of unit vectors in R?.
Now for any real number , define
[cos a - sina
Ra =
sin a
COS a
(a) Prove that for all a, B E R,
RaR3 = Ra+B
(b) Using part (a), or otherwise, prove that Ra is invertible and that R1
all a E R.
= R_a, for
(c) Prove that for all a E R and all x, y E R²,
(Rax) · (Ray) =x · y
(d) Suppose A is a 2 x 2 matrix such that for all x, y e R?,
(Ах) (Ау) — х у
Must it be true that A = Ra, for some a e R? Either prove this, or give a
counterexample (including justification).
(e) Let B =
be any 2 x 2 matrix.
(i) Show that there are real numbers u11 and a such that
[cos a
= U11
sin a
Hint: express
as a scalar multiple of a unit vector, and hence find an
expression for u11 in terms of a and c.
(ii) Let a e R. Use the invertibility of R. to prove that there are unique
U12, U22 E R such that
[cos a
= U12
sin a
sin a
+ U22
Cos a
(iii) Use parts (i) and (ii) to show that B can be expressed in the form
B = RU
for some a E R and some upper-triangular matrix U.
(iv) Suppose that B = RaU = RgV, where , B E R and U and V are upper-
triangular. Prove that if B is invertible, then U = ±V.
2
Transcribed Image Text:1. In this question, you will be using the following trigonometric identities: cos? a + sin? a (1) (2) (3) 1 cos(a + B) sin(a + B) cos a cos B – sin a sin B sin a cos B + cos a sin 3 where a, B E R. You do not need to prove these identities. You may also use without proof the fact that the set cos a : αER sin a is exactly the set of unit vectors in R?. Now for any real number , define [cos a - sina Ra = sin a COS a (a) Prove that for all a, B E R, RaR3 = Ra+B (b) Using part (a), or otherwise, prove that Ra is invertible and that R1 all a E R. = R_a, for (c) Prove that for all a E R and all x, y E R², (Rax) · (Ray) =x · y (d) Suppose A is a 2 x 2 matrix such that for all x, y e R?, (Ах) (Ау) — х у Must it be true that A = Ra, for some a e R? Either prove this, or give a counterexample (including justification). (e) Let B = be any 2 x 2 matrix. (i) Show that there are real numbers u11 and a such that [cos a = U11 sin a Hint: express as a scalar multiple of a unit vector, and hence find an expression for u11 in terms of a and c. (ii) Let a e R. Use the invertibility of R. to prove that there are unique U12, U22 E R such that [cos a = U12 sin a sin a + U22 Cos a (iii) Use parts (i) and (ii) to show that B can be expressed in the form B = RU for some a E R and some upper-triangular matrix U. (iv) Suppose that B = RaU = RgV, where , B E R and U and V are upper- triangular. Prove that if B is invertible, then U = ±V. 2
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