b=√3·c-a|b|²=|√3·c-a|²=(√3·c-a)·(√3·c-a)=3|c|²+|a|²-2√3·c·a∴2√3·c·a=3|c|²+|a|²-|b|²=3|c|²【∵|a|=|b|】∴cosθ=c·a/(|c|·|a|)=c·a/|c|²【∵|a|=|c|】=√3/2∴θ=π/6
