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If w is a non-real cube root of unity, x is a real number and `n in N` such that first three terms in the binomial expansion of `(w + x)^n` are `1,12bar w and 69 w` respectively, thenA. ` n = 36, x = 1`B. n = 12, x=2C. ` n = 24, x = (1)/(2)`D. ` n = 18, x = (1)/(3)` |
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Answer» We have , `( omega + x)^(n) = ""^(n)C_(0) omega^(n) x^(0) + ""^(n)C_(1) omega^(n-1) x + ""^(n)C_(2) omega^(n-2) x^(2) +…+ ""^(n)C_(n) x^(n)` `therefore omega^(n) = 1, n omega^(n-1) x= 12bar(omega) and ""^(n)C_(2) omega^(n -2) x^(2) = 69 omega` Now, `omega^(n) = 1 rArr n= 3 m, m in` N `therefore n omega ^(n-1) x = 12 bar(omega) and ""^(n)C_(2) omega^(n-2) x^(2) = 69 omega` `rArr 3m x = 12 and (3m (3m -1))/(2) x^(2) = 69` `rArr mx = 4 and 3m^(2) x^(2) - (mx) x = 46` `rArr mx = 4 and 3xx 16 - 4x = 46` `rArr x = (1)/(2) and m=8` `rArr x= (1)/(2) and n=24` [because n=3m]` |
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