The shear stressτin a shaft of diameter

D under a torque T is given by:

τ=kT

πD^3 .

Determine the approximate percentage error

in calculatingτif T is measured 3% too

small and D 1.5% too large.

(answer 7.5% decrease)

this is a question from J bird engineering mathematics book. it gives the answer but i need to know how to do the method.

any help much appreciated