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महाराष्ट्र राज्य शिक्षण मंडळएचएससी विज्ञान (सामान्य) इयत्ता ११ वी

If (x + iy)3 = y + vi then show that v(yx+vy) = 4(x2 – y2)

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प्रश्न

If (x + iy)3 = y + vi then show that `(y/x + "v"/y)` = 4(x2 – y2)

बेरीज
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उत्तर

(x + yi)3 = y + vi 

∴ x3 + 3x2yi + 3xy2i2 + y3i3 = y + vi

∴ x3 + 3x2yi + 3xy2 (–1) – y3i = y + vi  ...[∵ i2 = – 1, i3 = – i]

∴  x3 + 3x2yi - 3xy2 – iy3 = y + vi 

∴ (x3 – 3xy2) + (3x2y – y3)i = y + vi

Equating real and imaginary parts, we get

y = x3 – 3xy2 and v = 3x2y – y3

∴ `y/x` = x2 – 3y2 and `"v"/y` = 3x2 – y2

Consider,

L.H.S = `y/x + "v"/y`

= x2 – 3y2 + 3x2 – y2

= 4x2 – 4y2

= 4(x2 – y2)

= R.H.S.

Hence proved.

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