PROBLEM 18 : If two resistors with resistances R1 and R2 are connected in parall

PROBLEM 18 : If two resistors with resistances R1 and R2 are connected in parallel as shown in the figure below their electrical behavior is equivalent to a resistor of resistance R such that
1 / R = 1 / R1 + 1 / R2If R1 changes with time at a rate r = dR1/dt and R2 is constant express the rate of change dR / dt of the resistance of R in terms of dR1/dt R1 and R2. PROBLEM 19: Find a formula for the rate of change dV/dt of the volume of a balloon being inflated such that it radius R increases at a rate equal to dR/dt.
PROBLEM 20: Find a formula for the rate of change dA/dt of the area A of a square whose side x centimeter changes at a rate equal to 2 cm/sec.