GMATPrepNow wrote:
If line k passes through the points (48, 33) and (31, 22), what is the x-intercept of line k?
A) -25 10/11
B) -3
C) -1 16/17
D) 1 16/17
E) 25 10/11
*kudos for all correct solutions
Shorter approaches are listed above. If those methods are unfamiliar and you know the slope-intercept equation of a line, the latter can be used without shortcuts to find the x-intercept.
Given: the line passes through points (48, 33) and (31, 22)
• Slope-intercept equation of a line: \(y=mx+b\) \(m\) = slope and \(b\) = y-intercept. Find \(m\) and plug it into equation. Then find \(b\) and plug in. From that point find x-intercept.
• Slope = \(\frac{rise}{run}=\frac{y_2-y_1}{x_2-x_1}=\frac{33-22}{48-31}=\frac{11}{17}\) Equation of the line with slope plugged in: \(y=\frac{11}{17}x+b\)
• To find \(b\), plug in (x,y) values from one coordinate. Using (31,22)\(22=(\frac{11}{17}*31)+b\)
Clear the fraction*:
\((17*22)=(11*31)+17b\)
\((17*2*11) - (11*31)=17b\)
\(11(34-31)=17b\)
\(33=17b\)
\(b=\frac{33}{17}\)Equation with \(b\) plugged in: \(y=\frac{11}{17}x+\frac{33}{17}\)
• x-intercept? Set y equal to 0 in the equation.\(0=\frac{11}{17}x+\frac{33}{17}\)
\(0=11x+33\)
\(-33=11x\)
\(-\frac{33}{11}=x\)
\(-3=x\)Answer B
*When clearing the fraction, do not do the multiplication.
Number sense: 11 can be factored out . . . Or that fact will be clear when 11 in one term is next to 22 in the other on LHS.
If numbers seem awkward and/or huge, often it is smarter to leave them factored.