If a + b = 10 and ab = 21, then what is the value of a² + b² = ?

Question: If a + b = 10 and ab = 21, then what is the value of a² + b² = ?

Solution:
Given that,
a + b = 10
ab = 21

We know,
a² + b² =(a + b)² - 2ab
= (10)² - (2 × 21)
= 100 - 42
= 58