the diagonals bisect each other, then 1/2 the longer diagonal is 13 So, (13/34) represents 1/2 of the angle that the larger diagonal bisects. The angle would be approximately 2*67, or 134°
I m assuming the shorter diagonal is 26 units long.
The diagonals of a rhombus are at right angles and bisect each other so there will be a right angled triangle formed by the bisection with hypotenuse = 34 and adjacent side = 1/2 * 26 = 13
so cos x = 13/34 where x = 1/2 of the larger angle ( recall that the diagonals bisect the vertex angles in a rhombus)