1.

The value of `(.^(21)C_(1) - .^(10)C_(1)) + (.^(21)C_(2) - .^(10)C_(2)) + (.^(21)C_(3) - .^(10)C_(3)) + (.^(21)C_(4) - .^(10)C_(4)) + … + (.^(21)C_(10) - .^(10)C_(10))` isA. `2^(21) - 2^(11)`B. `2^(21) - 2^(10)`C. `2^(20) - 2^(9)`D. `2^(20) - 2^(10)`

Answer» Correct Answer - d
We have
`+( ""^(21)C_(1)-""^(10)C_(1))+( ""^(21)C_(2)-""^(10)C_(2))( ""^(21)C_(3)-""^(10)C_(3))`
`+( ""^(21)C_(4)-""^(10)C_(4))+...+( ""^(21)C_(10)-""^(10)C_(10))`
`= ( ""^(21)C_(1)+ ""^(21)C_(2)+ ""^(21)C_(3)+ ""^(21)C_(4) + ...+ ""^(21)C_(10))`
`= ( ""^(10)C_(1)+ ""^(10)C_(2)+ ""^(10)C_(3)+ ""^(10)C_(4) + ...+ ""^(10)C_(10))`
`=(1)/(2){ ""^(21)C_(1)+2""^(21)C_(2)+ 2""^(21)C_(3)+2""^(21)C_(4)+...+2 ""^(21)C_(10)}`
` -{ ""^(10)C_(1)+ ""^(10)C_(2)+ ""^(10)C_(3)+ ""^(10)C_(4) + ...+ ""^(10)C_(10)}`
` = (1)/(2) {( ""^(21)C_(1)+""^(21)C_(20))+ (""^(21)C_(2)+""^(21)C_(19))+(""^(21)C_(3)+""^(21)C_(18)) `
`+...+(""^(21)C_(10)+""^(21)C_(11))}={""^(10)C_(1)+""^(21)C_(2)`
`+""^(10)C_(3)+""^(10)C_(4)+...+ "^(10)C_(10)}`
`=(1)/(2) {""^(21)C_(1)+""^(21)C_(2)+""^(21)C_(3)+...+ ""^(21)C_(19) +""^(21)C_(20)}`
`-{ ""^(10)C_(1)+ ""^(10)C_(2) + ...+ ""^(10)C_(10)}`
`=(1)/(2) {(2^(21)-
(""^(21)C_(0)+""^(21)C_(21))}+{2^(10) - ""^(10)C_(0)}`
`=(1)/(2) {(2^(21)-2)(2^(10) -1) = 2^(20) - 2^(10)`.


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