lf t0,t1,t2,tn are the consecutive terms in the expansion (x+ai)n, then (t0−t2+t4−t6+….)2+(t1−t3+t5….)2=
A
x2+a2
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B
(x2+a2)n
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C
x2−a2
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D
(x2−a2)n
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Solution
The correct option is B(x2+a2)n (x+ai)n=nC0xn+nC1xn−1ai−nC2xn−2a2−nC3xn−3a3i+nC4xn−4a4+nC5xn−5a5i+..... (x+ai)n=(nC0xn−nC2xn−2a2+nC4xn−4a4+....)+i(nC1xn−1a−nC3xn−3a3+nC5xn−5a5) ⇒(x+ai)n=(t0−t2+t4+.....)+i(t1−t3+t5+.....) ⇒|(x+ai)n|=√(t0−t2+t4+.....)2+(t1−t3+t5+.....)2 .....(1) Now, (x−ai)n=nC0xn−nC1xn−1ai−nC2xn−2a2+nC3xn−3a3i+nC4xn−4a4−nC5xn−5a5i+..... (x−ai)n=(nC0xn−nC2xn−2a2+nC4xn−4a4+....)−i(nCn−1xa−nC3xn−3a3+nC5xn−5a5) ⇒(x−ai)n=(t0−t2+t4+.....)−i(t1−t3+t5+.....) ⇒|(x−ai)n|=√(t0−t2+t4+.....)2+(t1−t3+t5+.....)2 ....(2) Multiplying eqn (1) and (2), we get |(x+ai)n(x−ai)n|=(t0−t2+t4+.....)2+(t1−t3+t5+.....)2 ⇒(x2+a2)n=(t0−t2+t4+.....)2+(t1−t3+t5+.....)2