Show that sum k=1/2n(n+1) from k=1 to n

If rt is greater than ba, which statement must be true ?

Twelve less than a number is at most 20

A)a square brown tarp has a square green patch green in the corner the side length of the tarp is (x + 8) and the side length of the patch is x what is the area of the brown part of the tarpb)a square red placemat has a gold square in the corner the side length of the gold square is (x - 2) inches with the width of the red region is 4 in what is the area of the red part of the placemat