Answer:
x + 4x = 5x
Step-by-step explanation:
An identity is a mathematical equation that is always true (here - for all possible x's).
a). x + 4x = 5x; this is true, because for any possible x
x+4x = (1 + 4)x = 5x; there's not much to add.
b). 6x=18; evaluates to
x = 3; so it only works for one x, not all possible xs.
c). 2x + 1 = 7
2x = 6
x = 3; so it only forks for one x
d). 7x + 9 = x
6x + 9 = 0; we could compute the exact x it works for from here, but suffice to say this doesn't work for x = 0.
