Help #2 ​

3. 

a₁  = -4      a₂  =  8     a₃  =  -16

Note, NSS...it appears that we are multiplying each term by -2  to get the next term

So.....the "formula"  appears  to be

-4 * (-2)^n-1   

The  5th term  is     -4 *(-2)^{(5 - 1)}  =  -4 * (-2)⁴   =  -4 * 16   =   -64

4 .   This is a geometric series in disguise

The side of the the nth triangle  is given by

(50) * (.80)^{n - 1}

We could use some fancy math here...but let's just do this

Side  of  2nd trangle  =  50 * (.80)^{(2 - 1)}  = 50 * (.80)   =  45

Side of third triangle  =  45*.80  =  36

Side of 4th triangle  =  36 * .80  = 28.8 inches