Summaation of a sequence
or see a smaller scale
from 1 to 10
notice
1+10=11
2+9=11
etc
5+6=11
10/2=5
there are 5 pairs
5*11=55
so
1 to 100
1+100=101
2+99=101
...
50+51=101
100/2=50
there ar 50 pairs
50 times 101=5050
the sum is 5050
Answer: The ice sheet's thickness decrease every 6 weeks= 1.5 meters
Step-by-step explanation:
GIven: At the beginning of spring, the ice starts to melt. The variable sss models the ice sheet's thickness (in meters) t weeks after the beginning of spring. 
At t=0, 
So, Thickness of ice sheet at the beginning = 4 meters
Now at t= 6, we get

Thickness of ice sheet after 6 weeks = 2.5 meters
Decrease in thickness = 4 meters - 2.5 meters = 1.5 meters
Hence, the ice sheet's thickness decrease every 6 weeks= 1.5 meters
Obtuse angles measure more than 90 degrees, scalene angles have three different side lengths. Would you like for me to include angle measure in the sketch?