Question

If a+b = 10 and ab=21 find the value of a2+b2

Answer 1

ab = 21 -----(1)

a + b = 10

On squaring both sides, we get

a^2 + b^2 + 2ab = 100

=> a^2 + b^2 + 2(21) = 100

=> a^2 + b^2 + 42 = 100

=> a^2 + b^2 = 58

Answer 2

a+b=10
identity:(a+b)^2=a^2+b^2+2ab
so,
10^2=a^2+b^2+2(21)
100-42=a^2+b^2
=>a^2+b^2=58