Solve these simultaneous equations 5x + y = 21, x- 3y = 9
rearrange the first equation so it reads y= 5x+y= 21 y=21-5xSubstitute this y value into the second equation x-3y=9 y=21-5x x-3(21-5x)=9Expand out the bracket and rearrange to find out what x equals ( be careful with the changing plus and minus) x-63+ 15x=9 16x-63=9 16x=72 x=4.5REMEMBER you need to find x and Y so put your x value back into on of the original equations to find y-choose the easiest equation x=4.5 5x+y=21 5(4.5) +y=21 22.5+y=21 y= -1.5CHECK to check put both your values for x and y back into one of the original equations and make sure it works 5x+y=21 x=-1.5 y=4.5
5(-1.5) + 4.5 =21 so both x and y are correct
HW
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