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I need help on substitution.. I need help on 10 through 15

Algebra
Tutor: None Selected Time limit: 0 Hours

Nov 30th, 2014

10. 3x+y=7....(1)

       x=2y.......(2)

substitute x in (2)  in (1) above to get y

3(2y)+y=7

6y+y=7

7y=7

y=1

substitute y in (2) to get x

x=2y

x=2*1

x=2

answer x=2, y=1

11) 2x-5y=-19..........(1)

      y=x+2 ...........(2)

substitute y in (1)

2x-5y=-19

2x-5(x+2)= -19

2x-5x-10=-19

-3x-10=-19

-3x=-19+10

-3x=-9.............divide bothsides by -3

x=3

substitute x in (2)

y=x+2

y=3+2

y=5

answer x=3 ,y=5

12 4x+5y=44...(1)

    x+2=2y........(2)

in (2)        x+2=2y

                x=2y-2

substitute x in (1)

4x+5y=44

4(2y-2)+5y=44

8y-8+5y=44

13y=44+8

13y=52

y=4

substitute y in (2) to get x

x+2=2y

x+2=8

x=6

answer x=6, y=4

13 5x+15y=-3.......(1)

     x+3y=2  .........(2)

in (2) x=2-3y

substitute x in (1)

5(2-3y)+15y=-3

10-15y+15y=-3

y=0

substitute y in (2)

x+0=2

x=2

answer x=2 ,y=0

14 2x+y=-4  ..(1)

     x=y+7  ......(2)

substitute x in (1)

2(y+7)+y=-4

2y+14+y=-4

3y=-4-14

3y=-18

y=-6

substitute y in (2)

x=-6+7

x=1

Answer x=1, y=-6

15 4x+3y=-35...........(1)

    x+y=0...................(2)

(2) x=-y

substitute x in (1)

4(-y)+3y=-35

-4y+3y=-35

-y=-35

y=35

substitute y in (2)

x=-y

x=-35

answers x= -35, y=35

Dec 1st, 2014

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