Q. 155.0( 2 Votes )

# If (x – 3) and both are factors of ax2 + 5x + b, then show that a = b.

Let p(x) = ax2 + 5x + b

As (x – 3) and are factors of p(x) hence each will divide p(x) leaving remainder as 0

Let us use the remainder theorem which states that if (x – a) divides a quadratic polynomial p(x) then p(a) = 0

p(3) = 0

Substituting x = 3

a(3)2 + 5(3) + b = 0

9a + 15 + b = 0

b = – 9a – 15 …(ii)

And also

Substituting

Multiplying throughout by 9

a + 15 + 9b = 0

–9b = a + 15 …(i)

Add (i) and (ii)

b – 9b = – 9a – 15 + a + 15

–8b = –8a

a = b

Hence proved

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