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Express the following in the form of a + ib, a, b∈R i = −1. State the values of a and b: (2+i)(3-i)(1+2i) - Mathematics and Statistics

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Question

Express the following in the form of a + ib, a, b∈R i = `sqrt(−1)`. State the values of a and b:

`((2 + "i"))/((3 - "i")(1 + 2"i"))`

Sum
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Solution 1

`((2 + "i"))/((3 - "i")(1 + 2"i"))`

`= (2 + "i")/(3 + 6"i" - "i" - 2"i"^2)`

= `(2 + "i")/(3 + 5"i" - 2(-1))`  ...[∵ i2 = – 1]

= `(2 + "i")/(5 + 5"i")`

= `(2 + "i")/(5(1 + "i"))`

= `((2 + "i")(1 - "i"))/(5(1 + "i")(1 - "i"))`

= `(2 - 2"i" + "i" - "i"^2)/(5(1 - "i"^2)`

= `(2 - "i" - (-1))/(5[1 - (-1)]`  ...[∵ i2 = – 1]

= `(3- "i")/10`

∴ `(2 + "i")/((3 - "i")(1 + 2"i")) = 3/10 - 1/10"i"` 

∴ a = `3/10` and b = `(-1)/10`

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Solution 2

`((2 + "i"))/((3 - "i")(1 + 2"i"))`

`= (2 + "i")/(3 + 6"i" - "i" - 2"i"^2)`

= `(2 + "i")/(3 + 5"i" - 2(-1))`             ...[∵ i2 = – 1]

= `(2 + "i")/(3 + 5"i" + 2)` 

= `(2 + "i")/(5 + 5"i")`

= `((2 + "i").(5 - 5"i"))/((5 + 5"i").(5 - 5"i"))`

= `(10 - 10"i" + 5"i" - 5"i"^2)/(5^2 - 5"i"^2)`

= `(10 - 10"i" + 5"i" - 5(-1))/(5^2 - 5"i"^2)`              ...[∵ i2 = – 1]

= `(10 - 10"i" + 5"i" + 5)/(5^2 - 5"i"^2)` 

= `(15 - 5"i")/(25 - 25(-1))`  

= `(15 - 5"i")/(25 +25)`  

= `(15 - 5"i")/(50)`  

= `15/50 - (5"i")/50`  

= `3/10 - (1"i")/10` 

∴ write in a + ib form

∴ a = `3/10` and b = `(-1)/10`

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Chapter 1: Complex Numbers - Exercise 1.1 [Page 6]

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