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Standard form

SAT glossary · Reading & Writing

Definition

Ax+By=C; the form that hands you intercepts fast — set y=0 for the x-intercept C/A, set x=0 for the y-intercept C/B — with slope -A/B. The book warns that standard form hides the negative sign in the slope that slope-intercept form displays, which is why 3/4 appears as a distractor for -3/4.

Standard form in plain English

Standard form of a line, Ax+By=C, is especially handy for quickly finding BOTH intercepts without much work: set y=0 to instantly get the x-intercept, C/A, or set x=0 to get the y-intercept, C/B — and the slope works out to -A/B. The one thing to watch for: standard form HIDES a negative sign inside that slope formula that would be plainly visible in slope-intercept form — so a wrong-answer choice will often show up as frac34 when the real slope is actually -frac34, banking on you forgetting the built-in negative sign.

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Practice Standard form

A circle in the xy-plane is defined by the equation x^2 + y^2 - 10x + 6y + 18 = 0. What is the radius of the circle?

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