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Let $z = 3+i$ and $w = 1-i$ - HSC - SSCE Mathematics Extension 2 - Question 2 - 2005 - Paper 1

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Let-$z-=-3+i$-and-$w-=-1-i$-HSC-SSCE Mathematics Extension 2-Question 2-2005-Paper 1.png

Let $z = 3+i$ and $w = 1-i$. Find, in the form $x + iy$, (i) $2z + iw$, (ii) $zw$, (iii) $6/w$. Let $\beta = 1 - i\sqrt{3}$. (i) Express $\beta$ in mo... show full transcript

Worked Solution & Example Answer:Let $z = 3+i$ and $w = 1-i$ - HSC - SSCE Mathematics Extension 2 - Question 2 - 2005 - Paper 1

Step 1

Find, in the form x + iy, 2z + iw

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Answer

To solve for 2z+iw2z + iw, first substitute the values of zz and ww:

2z=2(3+i)=6+2i2z = 2(3+i) = 6 + 2i iw=i(1i)=i1iw = i(1 - i) = i - 1

Now, combine: 2z+iw=(6+2i)+(i1)=5+3i.2z + iw = (6 + 2i) + (i - 1) = 5 + 3i.

Step 2

Find, in the form x + iy, zw

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Answer

Now, calculate zwzw:

zw=(3+i)(1i)=3(1)+3(i)+i(1)+i(i)zw = (3+i)(1-i) = 3(1) + 3(-i) + i(1) + i(-i) =33i+i+1=42i.= 3 - 3i + i + 1 = 4 - 2i.

Step 3

Find, in the form x + iy, 6/w

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Answer

Next, compute 6/w6/w:

ightarrow ext{Multiply by the conjugate: } w' = 1 + i$$ $$6/w = 6/(1-i) = 6(1+i)/(1^2 + 1^2) = \frac{6(1+i)}{2} = 3 + 3i.$$

Step 4

Express β in modulus-argument form

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To find the modulus of β\beta:

β=12+(3)2=4=2.|\beta| = \sqrt{1^2 + (\sqrt{3})^2} = \sqrt{4} = 2.

For the argument, we have:

arg(β)=tan1(31)=π3.\arg(\beta) = \tan^{-1}\left(-\frac{\sqrt{3}}{1}\right) = -\frac{\pi}{3}.

Thus, in modulus-argument form: β=2cis(π3).\beta = 2\text{cis}\left(-\frac{\pi}{3}\right).

Step 5

Express β³ in modulus-argument form

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Answer

Using the modulus and argument found: β3=(2)3cis(3π3)=8cis(π)=8.\beta^3 = (2)^3\text{cis}\left(3 \cdot -\frac{\pi}{3}\right) = 8\text{cis}(-\pi) = -8.

Step 6

Hence express β³ in the form x + iy

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Answer

The form x+iyx + iy derived from β3\beta^3 is simply: β3=8+0i.\beta^3 = -8 + 0i.

Step 7

Sketch the region on the Argand diagram

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Answer

To sketch the inequalities zz<2|z - z| < 2 and z11|z - 1| \ge 1:

  1. The first inequality represents a region within a circle of radius 2 centered at zz.
  2. The second inequality represents the area outside or on the boundary of the circle of radius 1 centered at 11.

The intersection of these conditions should be marked distinctly on the Argand diagram.

Step 8

Explain why arg(z₁) + arg(z₂) = 2α

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Answer

When point PP is reflected across the line ll, the angle from the positive real axis to PP plus the angle from the positive real axis to QQ equals twice the angle of the line ll, which is 2α2\alpha. This is due to the nature of angle reflection in geometry.

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