## roberto5 2 years ago i need help Find the lowest common denominator for the fractions shown

1. roberto5

|dw:1352915488730:dw|

2. roberto5

i need help

3. roberto5

@jishan

4. roberto5

sure

5. jishan

yeah bro

6. Emah

17 goes into both 51 and 85...

7. roberto5

$8/51 19/85$ Find the lowest common denominator for the fractions shown

8. Emah

yeah but wouldn't you find a multiple that can go into both 51 and 85, then find the numerator...

9. jishan

51 factor=1,17,3 85factor=1,17,5 common=1

10. Emah

yeah i guess thats right... I thought he was trying to find the LCD...

11. jishan

yes sister

12. roberto5

A;4,335 b;255 c;17

13. roberto5

@phi

14. roberto5

@bloodrain

15. bloodrain

sorry im really busy and have to go soon (writing an english paper) hopefully someone else can help

16. phi

I need help to find the lowest common denominator of 51 and 85 the easiest way is to find the prime factors of each number to do this, you should learn (memorize) the first few (as many as you can manage!) prime numbers: 2 3 5 7 11 13 17 19 23 29 Here is a longer list http://www.mathsisfun.com/numbers/prime-numbers-to-10k.html 2nd, you should learn some rules for what numbers will evenly divide into another number. Try to learn these rules http://www.mathsisfun.com/divisibility-rules.html using the rule that if the sum of the digits is divisible by 3 then the number is divisible by 3 we figure out that 51 (digits 5+1=6 and 6/3 works) is divisible by 3. we get 3,17 From our list of prime numbers we know we have all the prime factors because 17 is prime now find the prime factors of 85 from the rule that if a number ends in 0 or 5 it is divisible by 5. we divide 5 into 85 to get 17 5*17 are the prime factors of 85 we have 3*17 and 5*17 to find the least common denominator we take a combination 3*5*17 ( notice that 3*17 divides into this number and so does 5*17) that is our common denominator now do your problem $\frac{8}{3\cdot 17}+\frac{19}{5\cdot17}$ you need to multiply the bottom of the 1st fraction by 5 to get the common denominator. And mult the bottom of the 2nd fraction by 3: $\frac{8}{3\cdot 17}\cdot \frac{5}{5}+\frac{19}{5\cdot17}\cdot \frac{3}{3}$ you get $\frac{8\cdot 5}{3\cdot 5\cdot 17}+\frac{19\cdot 3}{3\cdot 5 \cdot17}$ $\frac{8\cdot 5 +19\cdot 3}{3\cdot 5\cdot 17}$ multiply 5*8 to get 40 and 3*19 to get 57 $\frac{40 + 57}{3\cdot 5\cdot 17}$ add to get $\frac{97}{3\cdot 5\cdot 17}$ we should multiply out the denominator to get $\frac{97}{255}$