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Physics Guru is an online education and competitive examinations preparation portal founded by Manish Verma, an IIT Madras alumnus, providing learning resources for competitive examinations.

The emphasis is on conceptual understanding, analytical thinking and problem solving rather than simply memorising formulas and standard methods.

The objective is to help students understand what they are learning and develop the ability to solve problems they have not seen before.

Prove that, $a(a+b)+b(b+1)\geq -\frac {1}{3}$

To prove that $a(a+b)+b(b+1)\geq -\frac {1}{3}$ is same as proving that $a(a+b)+b(b+1)+\frac {1}{3}\geq 0$.

Consider, $a(a+b)+b(b+1)+\frac {1}{3}$

Or, $b^2+b(a+1)+a^2+\frac {1}{3}$

The above can be considered as quadratic expression in b, whose coefficient of $b^2$ is 1.

Discriminant $\Delta$=$(a+1)^2-4.\left( {{a^2} + \frac{1}{3}} \right)$

= $-3a^2+2a-\frac {1}{3}$

= $ - 3\left( {{a^2} - \frac{2}{3}a + \frac{1}{9}} \right)$

= $ - 3{\left( {a - \frac{1}{3}} \right)^2}$

$ \le 0$

Since coefficient of $b^2$ is 1 and discriminant is $ \le 0$ for the quadratic expression in b, the quadratic expression itself is $\ge 0$.