Bạn tham khảo!
Qua $C$ kẻ tia $Ch$ sao cho $Ch$ $//$ $AB$ $//$ $MN$
$→$ $∠A$ $=$ $∠C1$ $=$ $60^{o}$ (2 góc slt)
Vì $AB$ $//$ $MN$
$→$ $∠C2$ $=$ $∠M$ $=$ $30^{o}$ (2 góc slt)
Ta có : $∠ACM$ $=$ $∠C1$ $+$ $∠C2$ $=$ $60^{o}$ $+$ $30^{o}$ $=$ $90^{o}$
$FbBinhne2k88$