Identify Factors: To factor the quadratic expression x2−17x+72, we need to find two numbers that multiply to 72 (the constant term) and add up to −17 (the coefficient of the x term).
Find Correct Pair: We list pairs of factors of 72 and check which pair adds up to −17: 1 and 72 (1+72=73) 2 and 36 (2+36=38) 3 and 24 (3+24=27) 10 and 11 (12) 13 and 14 (15) 16 and 17 (18) We notice that none of the pairs add up to −17, but if we consider negative pairs, we find that 720 and 721 multiply to 72 and add up to −17.
Use Negative Pairs: Now we can write the quadratic expression as a product of two binomials using the numbers −8 and −9:x2−17x+72=(x−8)(x−9).
Write as Binomials: To check our work, we can use the FOIL method (First, Outer, Inner, Last) to expand the binomials and verify that we get the original expression:(x−8)(x−9)=x2−9x−8x+72=x2−17x+72.
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