If 4∑k=0(34−k(4−k)!)(xkk!)=323, then the largest value of x is
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Solution
4∑k=0(34−k(4−k)!)(xkk!)=4∑k=0(34−k(4−k)!)(xkk!)4!4!=4∑k=0(4!⋅34−kxk(4−k)!k!)14!=4∑k=04Ck⋅34−k⋅xk4!=(3+x)44!
Therefore, (3+x)44!=323⇒(3+x)4=28⇒(x+3)=±4 ⇒x=1or−7
So, the largest real value of x is 1.