Prima metoda:
Scriem ecuatia dreptei AB:
[tex]\frac{x-x_A}{x_B-x_A} =\frac{y-y_A}{y_B-y_A} \\\\\frac{x+1}{2+1} =\frac{y-1}{-2-1} \\\\-3(x+1)=3(y-1)\\\\-3x-3=3y-3\\\\-3x=3y\\\\y=-x[/tex]
Ca un punct M(a,b) sa apartina unei drepte atunci f(a)=b
In cazul nostru f(x)=-x
f(4)=a
a=-4
A doua metoda:
Determinantul=0
[tex]\left|\begin{array}{ccc}-1&1&1\\2&-2&1\\4&a&1\end{array}\right|=0[/tex]
-1 1 1
2 -2 1
2+2a+4-(-8-a+2)=0
6+2a+8+a-2=0
12+3a=0
3a=-12
a=-4
Un alt exercitiu cu ecuatia unei drepte gasesti aici: https://brainly.ro/tema/6037249
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