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Question


lf t0,t1,t2,tn are the consecutive terms in the expansion (x+ai)n, then

(t0t2+t4t6+.)2+(t1t3+t5.)2=

A
x2+a2
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B
(x2+a2)n
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C
x2a2
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D
(x2a2)n
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Solution

The correct option is B (x2+a2)n
(x+ai)n=nC0xn+nC1xn1ainC2xn2a2nC3xn3a3i+nC4xn4a4+nC5xn5a5i+.....
(x+ai)n=(nC0xnnC2xn2a2+nC4xn4a4+....)+i(nC1xn1anC3xn3a3+nC5xn5a5)
(x+ai)n=(t0t2+t4+.....)+i(t1t3+t5+.....)
|(x+ai)n|=(t0t2+t4+.....)2+(t1t3+t5+.....)2 .....(1)
Now, (xai)n=nC0xnnC1xn1ainC2xn2a2+nC3xn3a3i+nC4xn4a4nC5xn5a5i+.....
(xai)n=(nC0xnnC2xn2a2+nC4xn4a4+....)i(nCn1xanC3xn3a3+nC5xn5a5)
(xai)n=(t0t2+t4+.....)i(t1t3+t5+.....)
|(xai)n|=(t0t2+t4+.....)2+(t1t3+t5+.....)2 ....(2)
Multiplying eqn (1) and (2), we get
|(x+ai)n(xai)n|=(t0t2+t4+.....)2+(t1t3+t5+.....)2
(x2+a2)n=(t0t2+t4+.....)2+(t1t3+t5+.....)2

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