12. Simplify:
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a) (5+4i)(3+7i)(2−3i)
Step 1: Multiply the first two complex numbers.
(5+4i)(3+7i)=5(3)+5(7i)+4i(3)+4i(7i)
=15+35i+12i+28i2
Since i2=−1:
=15+47i−28
=−13+47i
Step 2: Multiply the result by the third complex number.
(−13+47i)(2−3i)=−13(2)−13(−3i)+47i(2)+47i(−3i)
=−26+39i+94i−141i2
Since i2=−1:
=−26+133i+141
=115+133i
The simplified expression is 115+133i.
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b) (4−3i)(2−3i)(3+2i)
Step 1: Multiply the complex numbers in the numerator.
(2−3i)(3+2i)=2(3)+2(2i)−3i(3)−3i(2i)
=6+4i−9i−6i2
Since i2=−1:
=6−5i+6
=12−5i
Step 2: Divide the result by the denominator.
4−3i12−5i
Multiply the numerator and denominator by the conjugate of the denominator, which is (4+3i).
(4−3i)(4+3i)(12−5i)(4+3i)
Step 3: Expand the numerator.
(12−5i)(4+3i)=12(4)+12(3i)−5i(4)−5i(3i)
=48+36i−20i−15i2
=48+16i+15
=63+16i
Step 4: Expand the denominator.
(4−3i)(4+3i)=42−(3i)2
=16−9i2
=16+9
=25
Step 5: Combine the numerator and denominator and express in a+bi form.
2563+16i=2563+2516i
The simplified expression is 2563+2516i.
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c) cosx+isinxcos3x+isin3x
Step 1: Use Euler's formula, eiθ=cosθ+isinθ.
cosx+isinxcos3x+isin3x=eixei3x
Step 2: Apply the rules of exponents for division.
ei3x−ix=ei(3x−x)=ei2x
Step 3: Convert back to trigonometric form using Euler's formula.
ei2x=cos(2x)+isin(2x)
The simplified expression is cos(2x)+isin(2x).
13. Express the following in the form a+ib.
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a) i(4−5i)2+3i+i2
Step 1: Simplify the first term, i(4−5i)2+3i.
First, simplify the denominator: i(4−5i)=4i−5i2=4i−5(−1)=5+4i.
So the term becomes 5+4i2+3i.
Multiply the numerator and denominator by the conjugate of the denominator, (5−4i):
(5+4i)(5−4i)(2+3i)(5−4i)=52−(4i)22(5)+2(−4i)+3i(5)+3i(−4i)
=25−16i210−8i+15i−12i2=25+1610+7i+12=4122+7i
=4122+417i
Step 2: Simplify the second term, i2.
Multiply the numerator and denominator by −i:
i2⋅−i−i=−i2−2i=−(−1)−2i=1−2i=−2i
Step 3: Add the simplified terms.
(4122+417i)+(−2i)
=4122+(417−2)i
=4122+(417−4182)i
=4122−4175i
The expression in the form a+ib is 4122−4175i.
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b) 2+3i1+1−2i1
Step 1: Simplify the first term, 2+3i1.
Multiply the numerator and denominator by the conjugate of the denominator, (2−3i):
2+3i1⋅2−3i2−3i=22−(3i)22−3i=4−9i22−3i=4+92−3i=132−3i
=132−133i
Step 2: Simplify the second term, 1−2i1.
Multiply the numerator and denominator by the conjugate of the denominator, (1+2i):
1−2i1⋅1+2i1+2i=12−(2i)21+2i=1−4i21+2i=1+41+2i=51+2i
=51+52i
Step 3: Add the simplified terms.
(132−133i)+(51+52i)
Group the real and imaginary parts: