Construction of a triangle given its perimeter and its two base angles


 
 
Concept Explanation
 

Construction of a triangle given its perimeter and its two base angles

Construction of Triangle Given Its Perimeter and Its Two Base Angles:  In order to construct a triangle of given perimeter and two base angles, we follow the following steps:

Steps of Construction:  Obtain the perimeter and the base angles of the triangle.Let ABC be a triangle of perimeter p cm and base BC.

STEP I  Draw a line segment XY equal to the perimeter p of large Delta ABC.

STEP II Construct dpi{100} large angle LXY=angle B  and large angle MYX=angle C.

STEP III : Draw bisectors of angles large angle LXY and large angle ;MYX and mark their intersection point as A.
STEP IV:  Draw the perpendicular bisectors PQ, RS  of XA and YA meeting XY in B and C respectively.
STEP V Join AB and AC to obtain the required triangle ABC.

Justification: For the justification of the construction, we observe that B lies on the perpendicular bisector of AX.

large therefore         XB = AB       large Rightarrow ;;angle AXB=angle BAX       [ Angles opposite to equal sides are equal]

Similarly, C lies on the perpendicular bisector of AY.

large therefore       YC = AC         large Rightarrow ;;angle AYC=angle YAC

Now,  XY = XB + BC + CY

large Rightarrow    XY = AB + BC + AC

In large Delta AXB, we

        large angle ABC=angle AXB+angle BAX           [Exterior angle is equal to the sum of interior opposite angle]

                             large =2angle AXB            [because the angles are equal Proved above]

                           large =angle LXB                  [ AX is the bisector ]

                            = large =angle B                      [By Step II of Construction]

In large Delta AYC, we have

           large angle ACB=angle AYC+angle YAC                 [Exterior angle is equal to the sum of interior opposite angle]

                             large =2angle AYC                      [because the angles are equal Proved above]

                            large =angle MYC                       [ AY is the bisector ]

                            large =angle C                                   [By Step II of Construction]

Example: Construct a triangle PQR whose perimeter is equal to 14 cm, large angle P=45^{circ} and large angle Q=60^{circ}.

SOLUTION  To draw large Delta PQR, we follow the following steps:

Steps of Construction:

STEP I  Draw a line segment XY = 14 cm

STEP II Construct large angle YXD=angle P= 45^{circ} and large angle XYE=angle Q= 60^{circ}

STEP III Draw the bisectors of angles large angle YXD and large angle XYE mark their point of intersection as R.
STEP IV  Draw right bisector AB, CD of RX and RY meeting XY at P and Q respectively.
STEP V Join PR and QR to obtain the required triangle PQR.

 

 
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