17. Bella took a square piece of paper and folded two of its sides to lie along the diagonal, as shown, to obtain a quadrilateral. What is the largest size of an angle in that quadrilateral?
A $112.5^{\circ}$
B $120^{\circ}$
C $125^{\circ}$
D $135^{\circ}$
E $150^{\circ}$
SolUTION
A

Since the quadrilateral is formed by folding the $45^{\circ}$ angles above and below the diagonal of the square in half, the size of the small angle of the quadrilateral is $2 \times\left(\frac{1}{2} \times 45^{\circ}\right)=45^{\circ}$. One angle of the quadrilateral is $90^{\circ}$ and the other two are equal from the construction. Therefore, since the sum of the angles in a quadrilateral is $360^{\circ}$, the size of the equal angles is $(360-90-45)^{\circ} \div 2=225^{\circ} \div 2=112.5^{\circ}$.