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

Answered by Harriet W. Maths tutor

2093 Views

See similar Maths GCSE tutors

Related Maths GCSE answers

All answers ▸

I feel really nervous about time pressure in the exam, I don't think I'll finish


Simplify: 2x + 6y + 2y - x


Factorise fully 2x^2 -x -4=2 and thus solve for x


Insert a pair of brackets into this question to make it correct 2 + 5 x -6 = -42


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo
Cookie Preferences