Which of the following does not represent a straight line?
y = x^2 + 7
x = 5y -1
y = (4 - x)/2
y = 4

in problems involving proportions, we can use cross products to test whether two ratios are equal and form a proportion. to find the cross products of a proportion, we multiply the outer terms, called the extremes, and the middle terms, called the means.

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