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Question:-1(b)

\(sin\left(\theta -\phi \right)=sin\:\theta \:cos\:\phi -cos\:\theta \:sin\:\phi \)
Express basis vectors \(e_1=(1,0)\) and \(e_2=(0,1)\) as linear combinations of \(\alpha_1=(2,-1)\) and \(\alpha_2=(1,3)\).
Expert Answer
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Introduction

The problem asks us to express the basis vectors e 1 = ( 1 , 0 ) e 1 = ( 1 , 0 ) e_(1)=(1,0)e_1 = (1, 0)e1=(1,0) and e 2 = ( 0 , 1 ) e 2 = ( 0 , 1 ) e_(2)=(0,1)e_2 = (0, 1)e2=(0,1) as linear combinations of the vectors α 1 = ( 2 , 1 ) α 1 = ( 2 , 1 ) alpha_(1)=(2,-1)\alpha_1 = (2, -1)α1=(2,1) and α 2 = ( 1 , 3 ) α 2 = ( 1 , 3 ) alpha_(2)=(1,3)\alpha_2 = (1, 3)α2=(1,3). In other words, we want to find constants c 1 c 1 c_(1)c_1c1 and c 2 c 2 c_(2)c_2c2 such that:
e 1 = c 1 α 1 + c 2 α 2 e 1 = c 1 α 1 + c 2 α 2 e_(1)=c_(1)alpha_(1)+c_(2)alpha_(2)e_1 = c_1 \alpha_1 + c_2 \alpha_2e1=c1α1+c2α2
e 2 = d 1 α 1 + d 2 α 2 e 2 = d 1 α 1 + d 2 α 2 e_(2)=d_(1)alpha_(1)+d_(2)alpha_(2)e_2 = d_1 \alpha_1 + d_2 \alpha_2e2=d1α1+d2α2

Work/Calculations

Step 1: Setting up the Equations for e 1 e 1 e_(1)e_1e1

The equation for e 1 e 1 e_(1)e_1e1 can be written as:
( 1 , 0 ) = c 1 ( 2 , 1 ) + c 2 ( 1 , 3 ) ( 1 , 0 ) = c 1 ( 2 , 1 ) + c 2 ( 1 , 3 ) (1,0)=c_(1)(2,-1)+c_(2)(1,3)(1, 0) = c_1 (2, -1) + c_2 (1, 3)(1,0)=c1(2,1)+c2(1,3)
Breaking it down into components, we get:
1 = 2 c 1 + c 2 1 = 2 c 1 + c 2 1=2c_(1)+c_(2)1 = 2c_1 + c_21=2c1+c2
0 = c 1 + 3 c 2 0 = c 1 + 3 c 2 0=-c_(1)+3c_(2)0 = -c_1 + 3c_20=c1+3c2

Step 2: Solving for c 1 c 1 c_(1)c_1c1 and c 2 c 2 c_(2)c_2c2

After Calculating, we get:
c 1 = 3 7 , c 2 = 1 7 c 1 = 3 7 , c 2 = 1 7 c_(1)=(3)/(7),quadc_(2)=(1)/(7)c_1 = \frac{3}{7}, \quad c_2 = \frac{1}{7}c1=37,c2=17

Step 3: Setting up the Equations for e 2 e 2 e_(2)e_2e2

The equation for e 2 e 2 e_(2)e_2e2 can be written as:
( 0 , 1 ) = d 1 ( 2 , 1 ) + d 2 ( 1 , 3 ) ( 0 , 1 ) = d 1 ( 2 , 1 ) + d 2 ( 1 , 3 ) (0,1)=d_(1)(2,-1)+d_(2)(1,3)(0, 1) = d_1 (2, -1) + d_2 (1, 3)(0,1)=d1(2,1)+d2(1,3)
Breaking it down into components, we get:
0 = 2 d 1 + d 2 0 = 2 d 1 + d 2 0=2d_(1)+d_(2)0 = 2d_1 + d_20=2d1+d2
1 = d 1 + 3 d 2 1 = d 1 + 3 d 2 1=-d_(1)+3d_(2)1 = -d_1 + 3d_21=d1+3d2

Step 4: Solving for d 1 d 1 d_(1)d_1d1 and d 2 d 2 d_(2)d_2d2

After Calculating, we get:
d 1 = 1 7 , d 2 = 2 7 d 1 = 1 7 , d 2 = 2 7 d_(1)=-(1)/(7),quadd_(2)=(2)/(7)d_1 = -\frac{1}{7}, \quad d_2 = \frac{2}{7}d1=17,d2=27

Conclusion

The basis vector e 1 = ( 1 , 0 ) e 1 = ( 1 , 0 ) e_(1)=(1,0)e_1 = (1, 0)e1=(1,0) can be expressed as a linear combination of α 1 α 1 alpha_(1)\alpha_1α1 and α 2 α 2 alpha_(2)\alpha_2α2 as follows:
e 1 = 3 7 α 1 + 1 7 α 2 e 1 = 3 7 α 1 + 1 7 α 2 e_(1)=(3)/(7)alpha_(1)+(1)/(7)alpha_(2)e_1 = \frac{3}{7} \alpha_1 + \frac{1}{7} \alpha_2e1=37α1+17α2
Similarly, the basis vector e 2 = ( 0 , 1 ) e 2 = ( 0 , 1 ) e_(2)=(0,1)e_2 = (0, 1)e2=(0,1) can be expressed as:
e 2 = 1 7 α 1 + 2 7 α 2 e 2 = 1 7 α 1 + 2 7 α 2 e_(2)=-(1)/(7)alpha_(1)+(2)/(7)alpha_(2)e_2 = -\frac{1}{7} \alpha_1 + \frac{2}{7} \alpha_2e2=17α1+27α2
Thus, both e 1 e 1 e_(1)e_1e1 and e 2 e 2 e_(2)e_2e2 can be represented as linear combinations of α 1 α 1 alpha_(1)\alpha_1α1 and α 2 α 2 alpha_(2)\alpha_2α2.
Verified Answer
5/5
\(cos\:3\theta =4\:cos^3\:\theta -3\:cos\:\theta \)
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