Question

Resolva esse sistema {x+y=280 e debaixo desta equação esta {5x+7y=1780

a)s=(80,160)
b)s=(40,80)
c)s=(90,190)
d)s=(70,150)
e)s=(60,120)​

Answer 1

$\large\boxed{\begin{array}{l}\begin{cases}\sf x+y=280\\\sf5x+7y=1780\end{cases}\\\begin{cases}\sf x=280-y\\\sf 5x+7y=1780\end{cases}\\\sf 5(280-y)+7y=1780\\\sf1400-5y+7y=1780\\\sf 2y=1780-1400\\\sf2y=380\\\sf y=\dfrac{380}{2}\\\sf y=190\\\sf x=280-y\\\sf x=280-190\\\sf x=90\\\sf S=\{(90,190)\}\\\huge\boxed{\boxed{\boxed{\boxed{\sf\maltese~alternativa~c}}}} \end{array}}$