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Find x when P = 21. P = x(x - 1) / 2
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Find x when P = 21.

P = x(x - 1) / 2

User Aniruddh Joshi
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21=(x(x-1))/(2)\\ 42=x^2-x\\ x^2-x-42=0\\ x^2+6x-7x-42=0\\ x(x+6)-7(x+6)=0\\ (x-7)(x+6)=0\\ x=7 \vee x=-6
User Paxal
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