Lucia finds that (3x^2+3x+5) + (7x^2-9x+8) = 10x^2-12x+13. What error did Lucia make?

The answer

to do the calculus, the method is

taking off the paranthesis

adding the term with a similar degree

summing constant terms

(3x^2+3x+5) + (7x^2-9x+8) =

= 3x^2+3x+5 + 7x^2-9x+8

= 3x^2 + 7x^2 + 3x+5 - 9x+8

= 10x² + 3x - 9x + 5 + 8

= 10x² - 6x + 13

the answer is

(3x^2+3x+5) + (7x^2-9x+8) = 10x² - 6x + 13

the error that Lucia did make 3x - 9x = - 12x, instead of 3x - 9x = - 6x