Question

The value of a,if the value of -x²+3x-a is equal to 8,when x=-1,is
(a)12
(b)14
(c)15
(d)-12​

Answer 1

Given

-x²+3x-a is equal to 8

To Find

value of a if the equation -x²+3x-a is equal to 8 at x =-1

$\sf\huge\bold{\underline{\underline{{Solution}}}}$

$\sf \longmapsto - {x}^{2} + 3x - a = 8$

$\sf \longmapsto {( - 1)}^{2} + 3( - 1) - a = 8$

Concept:

When there is negative value and and if it's powers is even e.g, 2,4,6,8 then the resultant would come be a positive value . Similarly,if value is negative and it's power is also -ve then the resultant be a negative value.

=> plus minus =minus

$\sf \longmapsto1 - 3 - a = 8$

$\sf \longmapsto - 2 - a = 8$

Now,here 2 is in -ve and when we shift it to right side it's sign gets changed from -ve to +ve

$\sf \longmapsto - a = 8 + 2 = 10$

$\sf \longmapsto - a = 10$

$\sf \bold{a = - 10}$

Check

$\sf \longmapsto {( - 1)}^{2} + 3( - 1) - ( - 10) = 8$

$\sf \longmapsto1 - 3 + 10$

$\sf \longmapsto - 2 + 10 = 8$

Answer 2

Answer:

Given

-x²+3x-a is equal to 8

To Find

value of a if the equation -x²+3x-a is equal to 8 at x =-1

\sf\huge\bold{\underline{\underline{{Solution}}}}

Solution

\sf \longmapsto - {x}^{2} + 3x - a = 8⟼−x

2

+3x−a=8

\sf \longmapsto {( - 1)}^{2} + 3( - 1) - a = 8⟼(−1)

2

+3(−1)−a=8

Concept:

When there is negative value and and if it's powers is even e.g, 2,4,6,8 then the resultant would come be a positive value . Similarly,if value is negative and it's power is also -ve then the resultant be a negative value.

=> plus minus =minus

\sf \longmapsto1 - 3 - a = 8⟼1−3−a=8

\sf \longmapsto - 2 - a = 8⟼−2−a=8

Now,here 2 is in -ve and when we shift it to right side it's sign gets changed from -ve to +ve

\sf \longmapsto - a = 8 + 2 = 10⟼−a=8+2=10

\sf \longmapsto - a = 10⟼−a=10

\sf \bold{a = - 10}a=−10

Check

\sf \longmapsto {( - 1)}^{2} + 3( - 1) - ( - 10) = 8⟼(−1)

2

+3(−1)−(−10)=8

\sf \longmapsto1 - 3 + 10⟼1−3+10

\sf \longmapsto - 2 + 10 = 8⟼−2+10=8