solve & graph the equations 3x - 6y = 12 2x - 5y = -10 To eliminate the y's: The coefficients of y in absolute value are 6 and 5. The smallest positive integer which both 6 and 5 will divide evenly into is 30. So we need to multiply each equation thru by whatever will cause one coefficient of y to be 30, and the other coefficient of y to be -30. There are two choices. We can either multiply the first equation thru by -5 and the second thru by +6 or the first thru by +5 and the second by -6. I'll arbitrarily choose to multiply the first one thru by +5 and the second thru by -6: 5[3x - 6y = 12] -6[2x - 5y = -10] Multiply thru, then add vertically term by term: 15x - 30y = 60 -12x + 30y = 60 ----------------- 3x = 120 x = 40 ------------------------------ 3x - 6y = 12 2x - 5y = -10 To eliminate the x's: The coefficients of x in absolute value are 3 and 2. The smallest positive integer which both 3 and 2 will divide evenly into is 6. So we need to multiply each equation thru by whatever will cause one coefficient of x to be 6, and the other coefficient of x to be -6. There are two choices. We can either multiply the first equation thru by -2 and the second thru by +3 or the first thru by +2 and the second by -3. I'll arbitrarily choose to multiply the first one thru by -2 and the second thru by +3: -2[3x - 6y = 12] 3[2x - 5y = -10] Multiply thru, then add vertically term by term: -6x + 12y = -24 6x - 15y = -30 ----------------- -3y = -54 y = 18 ---------------------------- x = 40 y = 18 So the two lines will cross at the point (40,18) --------------------------- To find the x-coordinate of the x-axis intercept, substitute 0 for y and solve for x. To find the y-coordinate of the y-axis intercept, substitute 0 for x and solve for y. For equation 1, 3x - 6y = 12 substitute 0 for y 3x - 6(0) = 12 3x - 0 = 12 3x = 12 x = 4 So the x-coordinate of the x-axis intercept is 4. The x-axis intercept is the point (4,0). That's the point at which the graph of equation 1 crosses the x-axis equation 1 x-axis intercept = (4,0) For equation 1, 3x - 6y = 12 substitute 0 for x 3(0) - 6y = 12 0 - 6y = 12 -6y = 12 y = -2 So the y-coordinate of the y-axis intercept is -2. The y-axis intercept is the point (0,-2). That's the point at which the graph of equation 1 crosses the y-axis. equation 1 y-axis intercept = (0,-2) For equation 2, 2x - 5y = -10 substitute 0 for y 2x - 5(0) = -10 2x - 0 = -10 2x = 10 x = 5 So the x-coordinate of the x-axis intercept is 5. The x-axis intercept is the point (5,0). That's the point at which the graph of equation 2 crosses the x-axis. equation 2 x-axis intercept = (5,0) For equation 2, 2x - 5y = -10 substitute 0 for x 2(0) - 5y = -10 0 - 5y = -10 -5y = -10 y = 2 So the y-coordinate of the y-axis intercept is 2. The y-axis intercept is the point (0,2). That's the point at which the graph of equation 2 crosses the y-axis. equation 2 y-axis intercept = (0,2) To graph the equations, get some points. You already have two on each line. Trouble is, the point where the lines cross has such large coefficients (40,18). The graph will have to be so big that we will need some other points since those are so small relative to the size we'll have to make the graph. So let's get some other points with bigger numbers: equation 1 x | y ------- 24 | 10 30 | 13 So we plot (24,10) and (30,13) and we get this lineThen for equation 2, we find a couple of points with large coefficients: equation 2 x | y ------- 20 | 10 35 | 16 So we plot (20,10) and (35,16) and we draw that line on the same set of axes and we see that they cross at the point (x,y) = (40,18). Edwin