Question

If two lines are intersecting as shown. At least, how many angles out of the four must be known in order to find all the angles?

• A. Zero
• B. One
• C. Two
• D. Three

Answer 1

The correct option is B

One

If no angle is known then definitely we cannot say anything about any of the angles, so zero cannot be the answer. Now let us start with one. Assume that any one of the angles is given say angle 'a'. Then angle 'c' will also be equal to 'a' because opposite angles at the point of intersection of two lines are equal. Secondly, sum of the angles at one side of a line,at the intersection of two lines is ${180}^{\circ }$. So, both the angles 'b' and 'd' will be '${180}^{\circ }$ - a'. Thus, we can determine all the angles with just one angle.