Make u the subject-
1) a=1/u + 1/2

u=2/2a-1

Step-by-step explanation:

a=1/u +1/2

you find the LCM first which is 2u

a=2+u/2u

cross multiply

2au=2+u

subtract u from both sides

2au-u=2

u is common to the right hand side

u(2a-1)=2

divide through by 2a-1

u=2/2a-1

the largest number of the four options is 1.8 x 10-2

you have it in scientific notation.

1.80 x 10-6   = 0.00000180

1.80 x 10-4   = 0.000180 (bigger, or "less small", than the former one)

so

1.8 x 10-2       = 0.0180. as you see. the "less small", or the biggest one of them all.

as you have negative exponent, it means you are dividing 1.8 by 100 (i.e. moving the dot towards left, two times).

if you had positive exponent for the scientific notation, power of tenths, you would be multiplying times 100 (moving the dot towards right) as below:

1.8 x (10)2 = 180

so. if you have negative exponent in scientific notation, move the dot toward left. if it is positive, toward right as many times the exponent tells you.