How many 4- letter words can be formed using the letters a, b, c, d, e and f, if
a) the word must start with a vowel? (cannot be repeated)

b) the word should start and end with a consonant?

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.

9987

step-by-step explanation:

i am not sure :

step-by-step explanation: