Mathematics

Problem

What is the correct expansion of

|\mathbf{a}+\mathbf{b}|^2

in terms of

|\mathbf{a}|

|\mathbf{b}|

and

\mathbf{a}\cdot\mathbf{b}

? a)

|\mathbf{a}|^{2}+|\mathbf{b}|^{2}+(\mathbf{a}\cdot\mathbf{b})^{2}

|\mathbf{a}|^{2}+|\mathbf{b}|^{2}+2\,\mathbf{a}\cdot\mathbf{b}

|\mathbf{a}|^{2}+|\mathbf{b}|^{2}-2\,\mathbf{a}\cdot\mathbf{b}

|\mathbf{a}|^{2}+|\mathbf{b}|^{2}+\mathbf{a}\cdot\mathbf{b}