方程组左对齐

$$ left{
egin{aligned}
{eta _1} &= {alpha _1}\
{eta _2} &= {alpha _2} - frac{{left( {{alpha _2},{eta _1}} ight)}}{{left( {{eta _1},{eta _1}} ight)}}{eta _1}\
&cdots cdots \
{eta _n} &= {alpha _n} - frac{{left( {{alpha _n},{eta _1}} ight)}}{{left( {{eta _1},{eta _1}} ight)}}{eta _1} - frac{{left( {{alpha _n},{eta _2}} ight)}}{{left( {{eta _2},{eta _2}} ight)}}{eta _2} - cdots - frac{{left( {{alpha _n},{eta _{n - 1}}} ight)}}{{left( {{eta _{n - 1}},{eta _{n - 1}}} ight)}}{eta _{n - 1}}
end{aligned}
ight.
$$