find the product 2/7a (ab – 7/6ab²) when a=2. b=-5
Answers
Answer:
-820 / 21
Step-by-step explanation:
2/7a (ab – 7/6ab²)
To solve this there are two methods
1. simplification and substitution
2. substitution and simplification
1. simplification and substitution:
2/7a (ab – 7/6ab²)
= 2/7 a²b - 2/7 x 7/6 a²b²
= 2/7 x 2² x ( -5) - 2/6 x 2² x ( -5) ²
= 2/7 x 4 x -5 - 1/3 x 4 x 25
= -40/ 7 - 100 / 3 LCM of 3 and 7 is 21
Making denominators equal we multiply -40 with 3 and 100 with 4
= ( -40 x 3 ) / 21 - ( 100 x 7) / 21
= -120 / 21 - 700 / 21
= ( -120 - 700 ) 21
= - 820 / 21
2. substitution and simplification:
2/7a (ab – 7/6ab²)
= 2/ 7 x 2 [ ( 2x -5 ) - ( 7/6 x 2 x ( -5)²) ]
= 4/7 [ -10 - ( 7/6 x 2 x 25)]
= 4/7 [ -10 - 175 / 3 ] multilpying -10 with 3 to make denominators equal
= 4/7 [ -30-175] / 3
= 4/7 [ - 205/ 3]
= ( 4 x - 205) / ( 7x3)
= - 820 / 21