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If x + iy = a+ibc+id, prove that (x2 + y2)2 = a2+b2c2+d2

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प्रश्न

If x + iy = `sqrt(("a" + "ib")/("c" + "id")`, prove that (x2 + y2)2 = `("a"^2 + "b"^2)/("c"^2 + "d"^2)` 

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उत्तर

x + iy = `sqrt(("a" + "ib")/("c" + "id")`

∴ (x + iy)2 = `("a" + "ib")/("c" + "id")`

∴ x2 + 2xyi + y2i2 = `("a" + "ib")/("c" + "id") xx ("c" - "id")/("c" - "id")`

∴ x2 + 2xyi – y2 = `("ac" - "adi" + "bci" - "bdi"^2)/("c"^2 - "d"^2"i"^2)`    ...[∵ i2 = –1]

∴ (x2 – y2) + 2xyi = `("ac" - "adi" + "bci" + "bd")/("c"^2 + "d"^2)` 

∴ (x2 – y2) + 2xyi =`(("ac" + "bd") + ("bc" - "ad")"i")/("c"^2 + "d"^2)`

∴ (x2 – y2) + 2xyi = `(("ac" + "bd")/("c"^2 + "d"^2)) + (("bc" - "ad")/("c"^2 + "d"^2))"i"`

Equating the real and imaginary parts separately, we get,

x2  – y2 = `("ac" + "bd")/("c"^2 + "d"^2)`  and 2xy = `("bc" - "ad")/("c"^2 + "d"^2)`

∴ (x2 + y2)2 = (x2 – y2)2 + 4x2y2

= (x2 – y2)2 + (2xy)2

= `(("ac" + "bd")/("c"^2 + "d"^2))^2 + (("bc" - "ad")/("c"^2 + "d"^2))^2`

= `(("ac" + "bd")^2 + ("bc" - "ad")^2)/("c"^2 + "d"^2)^2`

= `("a"^2"c"^2 + 2"abcd" + "b"^2"d"^2 + "b"^2"c"^2 - 2"abcd" + "a"^2"d"^2)/("c"^2 + "d"^2)^2`

= `(("a"^2"c"^2 + "b"^2"c"^2) + ("a"^2"d"^2 + "b"^2"d"^2))/("c"^2 + "d"^2)^2`

= `(("a"^2 + "b"^2)"c"^2 + ("a"^2 + "b"^2)"d"^2)/("c"^2 + "d"^2)^2`

= `(("a"^2 + "b"^2)("c"^2 + "d"^2))/("c"^2 + "d"^2)^2`

∴ (x2 + y2)2 = `("a"^2 + "b"^2)/("c"^2 + "d"^2)`

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अध्याय 1: Complex Numbers - Exercise 1.1 [पृष्ठ ६]

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