If `veca+vecb=vecc and a+b=c` , find the angle between `veca and vecb`.

Explanation

Let the angle between `vec a and vec b ` be `alpha`.
Here, `veca+vecb=vecc`
`therefore c^(2)=a^(2)+b^(2)+2ab cos alpha`........(1)
again, `a+b=c or, a^(2)+b^(2)+2ab=c^(2) .......(2)`
Hence from (1) and (2),
`a^2+b^2+2ab=a^2+b^2+2ab cos alpha`
`or, 2ab=2ab cos alpha or, cos alpha =1 = cos 0^@`
`therefore alpha = 0^@`