Answered

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If a + b = 7 and ab = 12 then what is the value of
a – ab – b ?

Sagot :

olumideolawoyin

Answer:

- 13 or - 11

Step-by-step explanation:

a + b = 7 . . . (1) and ab = 12 . . . (2)

Make b the subject of the formula,

b = 7 - a, then substitute in (2).

a(7 - a) = 12

7a - a² = 12 Re-arrange

a² - 7a + 12 = 0

By Factorization

a²- 3a - 4a + 12 = 0

a(a - 3) - 4(a - 3) = 0

(a - 3)(a - 4) = 0

a = 3 or a = 4

When a = 3,

b = 7 - 3

b = 4

When a = 4

b = 7 - 4

b = 3

Therefore, when a = 3, b = 4

a – ab – b = 3 - 3×4 - 4

= 3 - 12 - 4

= - 13

when a = 4, b = 3

a – ab – b = 4 - 4 × 3 - 3

= 4 - 12 - 3

= - 11

Therefore,

a – ab – b = - 13 or - 11

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