Algebraic expressions..
Evaluate 3x-y2 for x = -3/4 and y = -2/5

- anonymous

Algebraic expressions..
Evaluate 3x-y2 for x = -3/4 and y = -2/5

- katieb

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- welshfella

you replace x by -3/4 and y by -2/5 in the expression and work it out.
= 3*(-3/4) - (-2/5)^2

- anonymous

I know i plug it it in so it would be 3(-3/4) - (-2/5)^2
right?

- welshfella

yes

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## More answers

- anonymous

i dont like working with fractions i always get confused

- welshfella

multiplying fractions is not too difficult
just multiply top and bottom:-
|dw:1440933535144:dw|

- welshfella

* thats 15 not 35!!

- anonymous

is there more steps?

- welshfella

yes do the multiplication and squaring first
finally subtract

- anonymous

hmm

- welshfella

|dw:1440933718810:dw|

- anonymous

the first one was just an example right?

- welshfella

yes

- anonymous

ok :)

- anonymous

do i multiply the 4 and -3

- anonymous

and then add the 3

- welshfella

no - multiply the top numbers that is -3 and 3

- anonymous

oh okay!

- anonymous

okay i got 9/4 - 4/25 is it wrong?

- welshfella

no that is correct

- anonymous

and then i subtract?

- anonymous

or multiply

- anonymous

the answer on the worksheet says its -2 41/100|dw:1440934161182:dw|

- welshfella

notice that 9/4 is an improper fraction I ts easier at this stage to convert iutto a mixed number
9 divide by 4 is 2 and there is a remainder 1
so 9/4 = 2 1/4

- anonymous

oh!

- welshfella

so we have
|dw:1440934184640:dw|

- anonymous

got it

- welshfella

sorry it was negative -9/4 - 4/25 not 9/4 - 4/25

- anonymous

its okay

- welshfella

now you can only subtract and add fractions withe the same denomiantor so you need to find the LCD (Lowest Common Denominator) of 4 and 25

- welshfella

LCD of 4 and 25 is 100

- anonymous

yes it is

- welshfella

so converting 1/4 to hundredths we multiply the top ond bottom of the fraction by 15
|dw:1440934433350:dw|

- anonymous

does the 2 dissapear?

- welshfella

and
|dw:1440934510629:dw|

- welshfella

No we'll bring that in at the end

- welshfella

so we have now got
|dw:1440934591025:dw|

- anonymous

OH!! I understand now :) thanks so much!

- welshfella

good Try and practice some more problems of same type and you'll gain confidence

- welshfella

fractions are a bit tricky

- anonymous

yes i will thank you!

- welshfella

yw

- anonymous

okay i just tried figuring out how to get 41 do you subtract the 25 and 10?

- welshfella

- and watch the negatives - i forgot them a few times myself!

- welshfella

no -16 - 25 = -41

- anonymous

you add 2 to 25 and add 2 to 10? and that gives you 39? and -2 + 41 = 39

- anonymous

oh

- anonymous

i thought that was a 10 sorry

- welshfella

you are really adding negative numbers -16 + (-25) = -41
Tha'ts what makes this one extra tricky - you have negatives and fractions..

- welshfella

- thats my bad drawing

- anonymous

Okay i just checked my work :)

- welshfella

lets try -3/16 - 7 / 16
what does that equal?

- welshfella

-3 - 7 = ?

- anonymous

-10?

- welshfella

yea so its -10/16
when add/subtract the denominator stays the same

- anonymous

yeah i always get confused by negatives

- welshfella

Practice

- anonymous

Thanks again :)

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