Form 3 Maths 幫幫手 － Q 4

d) If Patrick walk 60cm backward...

Let new angle of elevation from Patrick's eye to Alfred's head be α, then

tan α = 18/(60+60) = 0.15

new distance y from Patrick's eye to Alfred's head at the ruler on the wall

tan α = y/(60+60+10)

y= 0.15 x 130 = 19.5

Therefore,

Patrick read about Alfred's head at the ruler on the wall = 138 + 19.5 = 157.5 =158cm (correct to nearest cm)