If 3,a-1,a+1 and 5 are pproportional, then a =?

a = ± 4

Step-by-step explanation:

Expressing the proportion in fractional form

= ( cross- multiply )

(a -1 )(a + 1) = 15 ← expand factors on left side

a² - 1 = 15 ( add 1 to both sides )

a² = 16 ( take the square root of both sides )

a = ± = ± 4

36 is the final answer! : )