For all integer values of x and constant k, if (x-k)(x-5)=x^2-9x+20, what is the value of K? A) -5
B) -4
C) 4
D) 10
E) 12

Work Shown:

Expand out the left hand side

(x-k)(x-5)

x(x-5)-k(x-5)

x^2-5x-kx+5k

Note that the constant term here is 5k. Yes k seems like a variable, but it's actually a constant. Once we know k, we can replace it to get a fixed number. In this case, k = 4 since 5k = 5*4 = 20 to have it match with the 20 at the end of x^2-9x+20

For the x terms, we have -5x-kx = -5x-4x = -9x which matches with the middle term of x^2-9x+20

Therefore,

(x-k)(x-5) = x^2-9x+20

updates to

(x-4)(x-5) = x^2-9x+20

The -4 and -5 multiply to 20, and they also add to -9. This helps confirm we have the right k value.

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step-by-step explanation: