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Sum of roots

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2 Answers

If a(x-b)(x-c)=0,
then ax^2 - a(b+c)x + bc = 0.
The sum of the two roots is the opposite of the coefficient of the linear term divided by the leading coefficient.
Tom's 3rd line is 2x^2 - 7x -15 = 0.
We can immediately say the sum of the roots is 7/2.