Delhi Police Constable Exam Model Paper 6

Question : 87 of 100

Marks: +1, -0

If a + b = 9 and ab = 20, then find the value of (

a^3+ b^3).

Solution:

Given:
A + b = 9 and ab = 20
Formula:

a^3 + b^3 = (a + b)^3 – 3ab(a + b)

Calculation:

a^3 + b^3 = (a + b)^3 – 3ab(a + b)

⇒

a^3 + b^3 = 9^3 – 3 × 20 × 9

⇒

a^3 + b^3 = 729 - 540

⇒

a^3 + b^3 = 189

Smart Trick
Take a = 5 and b = 4 (a + b = 5 + 4 = 9 and ab = 5 × 4 = 20)
∴

a^3 + b^3 = 5^3 + 4^3 = 125 + 64 = 189.

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