First, we {{CMD_172}}introduce variables{{CMD_173}} {{MATH_174}}, the distance the plane has traveled after it flew right above you, and {{MATH_175}}, the distance between you and the plane. The given rate is {{MATH_176}} mph and the unknown rate is {{MATH_177}}, when {{MATH_178}}. Next, we {{CMD_179}}draw a picture{{CMD_180}}. {{CMD_181}} {{ENV_182}} {{CMD_183}} Next we {{CMD_184}}find equations{{CMD_185}} relating the variables {{MATH_186}} and {{MATH_187}}. By the Pythagorean Theorem we know that {{MATH_188}} Since {{MATH_189}} and {{MATH_190}} are functions of time, we now {{CMD_191}}differentiate{{CMD_192}} both sides of the equation {{MATH_193}} At the instant when {{MATH_194}} mi, we get that {{MATH_195}} This implies that {{MATH_196}} has to be evaluated at time when {{MATH_197}}. In order to do that, let's draw the picture at the given instant, when {{MATH_198}} mi. {{CMD_199}} {{ENV_200}} {{CMD_201}}