Krishna
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Step 1: According to the given data construct an imaginary figure for better understanding.

              EXAMPLE:


Step 2: Make sure that all the measurements have the same units or not

            NOTE: Convert the measurements into same units

            EXAMPLE: Height of a girl = 90 cm

                                height of a lamp = 3.6 m = 3.6 * 100 =  360 cm

                                The girl walking speed = 1.2 m/sec = 120 cm/sec


Step 3: Find the distance travelled by the girl  

            NOTE: Distance = speed * time

            EXAMPLE: From the figure

                              Distance (QT) = 120 \frac{cm}{sec} * 4 sec

                                                QT = 480 cm


Step 4: Prove that the required triangles(PQR & STR) are similar

          EXAMPLE: From the figure  (PQR, STR)

                             \angle Q = \angle T = 90

                            And we have a common angle for two triangle that is

                               \angle R

            So, PQR  ~ STR (AAA similarity theorem)


Step 5: Use the AAA theorem to calculate unknown value

        AAA THEOREM: In two triangles, if the angles are equal, then the sides

        opposite to the equal angles are in the same ratio (or proportional) and hence

        the two triangles are similar. 

             \frac{PQ}{QR} = \frac{ST}{TR}


Step 6: Find the length of the shadow

            EXAMPLE: \frac{PQ}{QT + TR} = \frac{ST}{TR}

                Substitute the known values

                   \frac{360}{480 + TR} = \frac{90}{TR}

                 360*TR = (480 + TR)* 90

                            270*TR = 480*90

                       TR = \frac{43200}{270}

                                  TR = 160 cm

              The length of the shadow = 160 cm